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Margaret Hamilton was the Apollo Director of Software Engineering. In this picture, she is shown inside a mock-up of the Apollo command module.

Hamilton is noted for many accomplishments during the Apollo program. She led the team that wrote all of the software for the guidance computers, invented the term "sofware engineering", and insisted on unit and integration tests of all software. Her work was critical to solving computer problems during Apollo 8 and Apollo 11. In 2016, Hamilton was awarded the Presidential Medal of Freedom (the highest U.S. civilian honor) for her contributions to the Apollo program and software engineering.

She also personally wrote many of the routines for the DSKY display and keypad unit, seen above her head in the picture. It is therefore odd that she had posed with her hand on an unrelated control (apparently a toggle switch or a circuit breaker), instead of the DSKY.

Can anyone identify the control that her hand is on in this picture?

It's the ECS RADIATORS / HEATER / PRIM 1 - PRIM 2 switch.

from here

Having given many tours of the cockpit in the shuttle simulators, it's very common to have someone say "Turn around for the camera and put your finger on a switch!"

Organic Marble beat me to it, but in the interest of teaching people to fish:

There's a handy panel locator figure which subdivides the command module control panel into several lettered areas as well as a grid reference. This switch is in area "P", grid J-34 on panel 2.

Looking up that location in the table near the start of the controls and displays section of the Apollo operations handbook identifies it as the "ECS RADIATORS-HEATER - PRIM 1/CENTER/PRIM 2" switch.

This is part of the spacecraft's environmental controls.

Reading further in the (regrettably, not text searchable) handbook, the PRIM 1 position "selects primary heater control no. 1"; CENTER "disables [primary] heater controllers"; PRIM 2 "selects primary heater control no. 2". It's not entirely clear from this document what selecting the heater control actually does; Organic Marble is almost certainly correct that the choice of switch was photo-op arbitrary.