Which compound had the smallest Rf in a solution of ethyl acetate/hexanes? With reference to the structures, explain this result in terms of intermolecular forces.

a) (CH₃)₂CHCH(CH₃)₂
b) BrCH₂CH₂CH₂Cl
c) BrCH₂CH₂CH₂Br
d) CH₃CH₂CH₂CH₃