Why is the answer to gravitational potential energy question C?

[jack1234]

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

 

[Sourabh N]

What do you think about gravitational potential energy of a cubical block?

 

[jack1234]

I am thinking about centre mass...
So what I am figuring is
3m*g*(3/2 l)=9/2mgl...(3/2 l because the centre of mass is at that height)
Is it correct?

 

[Sourabh N]

perfect.