Lucy Wilson Benson, former president of the League of Women Voters of the United States, has been selected to be under secretary of state for security assistance in the Carter administration.

Benson, whose nomination must be confirmed by the Senate, would be the highest-ranking woman to serve in a State Department policymaking post. The job was created in 1971 primarily to handle foreign military aid. Sources close to the transition said Benson also will be given responsibility for armaments transfers and efforts to combat proliferation of nuclear weapons.

Benson, 49, has been on a wide variety of civic and corporate boards and recently served as Secretary of Human Affairs of the state of Massachusetts.During her term as League of Women Voters president, the organization became more youth oriented and took a stand of "deep concern" about President Nixon's 1970 invasion of Cambodia.

The future appointee, who met yesterday with Secretary of State-designate Cyrus R. Vance, has limited experience in foreign or military policy. Shw was a member of the Trilateral Commission, from which President-elect Jimmy Carter has taken several of his future nominees in the foreign policy field.

In Plains, Ga., Carter officially announced his plan to name Los Angeles attorney Warren M. Christopher to be deputy secretary of state, the No. 2 State Department post. The announcement said Christopher had been picked "on the advice" of Vance, his future boss.

Richard Moose, staff director of the Foreign Assistance subcommittee of the Senate Foreign Relations Committee and one of the architects of recent legislation on arms sales abroad, has been picked to become deputy under secretary of state for management, according to informed sources. Moose previously served on the National Security Council staff and as a roving investigator for the Senate Foreign Relations Committee. He has recently worked as a transition aide for the Carter administration at the State Department.

The management post, presently occupied by Secretary of State Henry A. Kissinger's close aide, Lawrence S. Eagleburger, has authority over State Department personnel and career service functions. Moose, while a National Security Council staff member under Kissinger, was among those whose telephones were tapped by orders of high officials.

Carter's future White House national security adviser, Zbigniew Brzezinski, plans to reduce the NSC staff from about 45 professionals at present to around 30 or fewer and 10 reorganize the staff along functional lines, knowledgeable sources said.

Carter has not yet given final approval to the reorganization or to Brzezinski's personnel selections, the sources said. However, Brzezinski is reported to be determined to carve out "a coordinating role" that will be "a stimulus for foreign policy" rather than become an operator and negotiator in the Kissinger tradition.

David Aaron, a former member of the Senate intelligence committee staff and an assistant Vice President-elect Walter F. Mondale, has been selected to be Brzezinski's NSC deputy. Aaron has been serving as a transition aide at the NSC.

Only five to 10 of the present NSC professional staff members are likely to remain after the transition is completed, sources said. Among those expected to stay is Robert Hormats, who has been in charge of economic matters.

Brzezinski is reported to be hoping that Michel Oksenberg, associate professor of plitical science at the University of Michigan and a prominent China scholar, will join the NSC as a staff member dealing with Asia.