The answer is (c); the mass must be positioned at 2/3 of the distance from the pivot point of the physical pendulum, as seen on an mpeg video by clicking your mouse on the photograph below. The video goes fast and is sort of blurry, so you may have to grab the little doodad at the bottom of the screen and scroll the video more slowly.
This is the location of the radius of gyration of the physical pendulum - the length of a simple pendulum with its period equal to that of the physical pendulum. Try comparing the periods of a physical pendulum and a simple pendulum with 2/3 the length of the physical pendulum.
