The two most widely used investment funds in the 401(k)-style retirement savings plan for federal and postal employees posted modest gains in 2011, while the other investment options finished the year with widely varying results.

The Thrift Savings Plan reported Tuesday that its most popular investment option, a fund of special-issue government securities called the G fund, gained 2.45 percent for the just-ended calendar year, while a fund that tracks the Standard & Poor’s 500 index of large U.S. company stocks, called the C fund, rose 2.11 percent.

The G fund held about 45 percent of investor assets while the C fund held 23 percent through November, the latest month for which participant figures are available. Through that month, there were about 4.5 million investors in the TSP with about $292 billion in their accounts.

The TSP is open to federal and postal employees as well as to active-duty military personnel. Investors who retire or separate for other reasons may choose from several withdrawal options or may keep their accounts open.

The largest gain in 2011 was posted by a mixed corporate-government bond fund, which rose 7.89 percent. Meanwhile, a fund tracking smaller company U.S. stocks dropped 3.38 percent, and one tracking an international stock index fell 11.81 percent.

Overall, investors became slightly more conservative in their account allocations during the year, which saw high volatility in stock investing and a steep drop in stock values during the summer. At year-end 2010, the G fund had held only about 42 percent of investor assets, while the C fund held 25 percent. The combined total of the two other stock-oriented funds fell from 16 to 13 percent in that time.

The TSP also offers “life cycle” funds that mix investments in the five main funds, in proportions varying according to the projected withdrawal date. The most popular of those, the target date 2020 fund, gained 0.41 percent last year, while the target date 2030 and 2040 funds fell by 0.31 and 0.96 percent, respectively. The TSP did not calculate a 2011 return for a fund newly launched during the year with a 2050 target date, while an income fund for those already making withdrawals or who plan to do so soon gained 2.23 percent.

About 2.6 million federal and postal employees have TSP accounts, along with about 700,000 military personnel. The remaining accounts are held by retired or otherwise separated persons; they may continue to manage their investments but cannot add money to their accounts.