Gravitational potential energy?

Question

The answer is c, but I can't figure out what is the reason, can somebody kindly explain it?
http://tinyurl.com/32322t

Answer 1

The centre of mass of a cube is half way along its length.

So the PE of the first cube is mg.l/2

The second is l higher, so its mg.(1 + 1/2)l

The third is l higher again, sot its mg.(2 + 1/2)l

Add.

Answer 2

Gravitational potential is defined as the Ratio of Gravitational energy divided by the mass which is located at that particular point in the Gravity field.