House Republicans, in a fund-raising letter and questionnaire, have suggested that making Social Security voluntary and raising the retirement age are possible options for putting the system back on a sound financial footing.

The same letter, obtained yesterday by The Washington Post, says cutting the annual cost-of-living increase also is a leading option.

The letter, outlining highly controversial concepts that have been fiercely opposed by organizations of the aged and Social Security system experts, was sent out by the National Republican Congressional Committee, the official campaign committee for House Republicans. It was signed by Chairman Guy Vander Jagt (R-Mich.).

In the past, President Reagan has raised the possibility of making Social Security voluntary, which many experts believe could destroy the system, but Reagan sharply retreated from that position in the 1980 campaign.

Asked for comment on the fund-raising letter, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said in a statement released here, "You do not build confidence in Social Security by polling people on how to dismantle it. It does not surprise me that Republicans are plotting to destroy Social Security. What surprises me is how blatant they can be about it."

Rich Galen, a spokesman for the House GOP campaign group, said the letter was "not a proposal, just a list of options." Calling the O'Neill statement "campaign hyperbole," Galen said, "Of course we don't want to destroy the system."

Vander Jagt, appealing for funds, said in the letter that Reagan wants to save the system but the Democrats "began dipping" into it "to fund more of their pet social welfare schemes, and weighing it down with more and more beneficiaries who never paid a dime into the system."

The letter included three ballots, each with a different option for solving Social Security's financial problems. Contributors were asked to return the ballot they preferred, with a check.

The first ballot said "Making Social Security Voluntary."

The second ballot said "Split Off the Welfare Aspects of Social Security." In smaller print, this option is described as "offering benefits only to those who pay into the system and their survivors."

This presumably could mean important shifts in the Social Security benefit structure, which now is tilted deliberately and rather sharply in favor of lower-income workers. So-called replacement rates are much higher for lower-paid workers than for those in the middle and upper pay ranges. This means their first benefit checks are larger in relation to their last paychecks than is true at the upper ends of the pay scale.

The third ballot said "Adjust the Financing of Social Security without Making Any Fundamental Change in Its Structure." In smaller print, the ballot explains that possible ways to do this would be "reducing the cost-of-living allowance (COLA) for current beneficiaries; raising Social Security taxes; tightening eligibility for Social Security; requiring federal employes to participate in Social Security; raising the retirement age for future retirees."

Galen said 400,000 of the letters were sent out early in September, and 40,000 ballots were returned, only 10 percent with checks. He said 10,000 people voted for the first option, 25,000 for number two, and 5,000 for number three.