Question: My mother, age 65 and retired, will soon have $100,000 to invest from the sale of my recently deceased father's business. She wants her investments to provide regular income, since her only other source of income is Social Security. How would you suggest investing the money to maximize return?

Answer: There are several places your mother could put the funds to provide her with a regular income at minimal risk.

Money market funds are paying about 17 percent now. (All the yield figures I cite here were those available in early July.) The rate will vary as market interest rates fluctuate. Although the present yield may be a peak, rates are expected to ramain rather high for some time.

A unit trust such as Merrill Lynch's long-term Corporate Income Fund can now be bought to yield 15 percent, payable monthly and assured for the life of the trust, generally 17 to 18 years.

If your mother wants the additional security of a government guarantee, she can look at Government National Mortgage Association mortgages. Although the depressed real estate market has limited the amount of real estate mortgages available, GNMA unit trusts are yielding in the neighborhood of 14 percent.

Or she can buy an annuity, which will give her a guaranteed income that she can't outlive. Payouts vary among companies and she should do some comparison shopping. But she should be able to get something like $900 a month for life in return for a $100,000 deposit.

The problem with an annuity is that on her death, the principal sum is retained by the insurance company. Depending on the policy she buys, there may be guaranteed payments to a beneficiary, up to some specified minimum amount, if she were to die in the early years of the program.

All the investments I have mentioned have a common drawback: there is no opportunity for appreciation of the principal to compensate for future inflation. The yields on unit trusts and fixed annuities are set at inception and will remain unchanged for the life of the trust.

Your mother could buy a variable annuity that offers the possibility of changes in the monthly payout as the return on the underlying portfolio of common stocks changes. But I haven't been impressed with the past performance of these annuities.

Social Security payments are tied to changes in the cost-of-living index. I expect this principle to remain intact, although there may be changes in the index used or in the method of computing the annual increase.

But since the income from the $100,000 will constitute the larger part of your mother's income, I would put perhaps 25 percent of the total available into an inflation-sensitive investment.

That could be an income-oriented mutual fund. You can get a directory of noload (no fee) funds for $1 from the NoLoad Mutual Fund Association, Valley Forge, Pa. 19481. Or a free listing of all member funds, load and no-load, is available from the Investment Company Institute, P.O. Box 19999, Washington, D.C. 20036.

As an alternative, she might consider buying 100 shares in each of two or three utilities. A broker can help her select companies with good financial strength and with a record of regular dividend increases. She should be able to get 12 or 13 percent in current dividends.

Q. It is permissible to treat the discount on a Treasury bill as income for tax purposes in the year in which it is earned? For example, if I buy a 180-day bill on Dec. 11, 1980, due June 1981, can I report 20 days of interest on my 1980 tax return and the balance in 1981?

A. No. Original issue discount on a Treasury bill is considered interest income that is reported on your tax return for the year in which the bill matures and is redeemed.

This is a different rule than for corporate bonds issued at a discount (after May 27, 1969). In this case, the original issue discount is a sweetener to induce investors to buy the bonds and is really equivalent to offering a higher yield.

But the T-bill discount is simply the government's bookkeeping method of computing and paying the interest. In line with the general rules for income for cash-basis taxpayers, it is reported in the year actually received.