Which compound would show optical isomerism?

Explanation

In CH3CHClCH2CH3 the second carbon carries a methyl group, a chlorine, a hydrogen and an ethyl group, four different substituents, so it is chiral and the molecule exists as two non superimposable mirror images. In every other option at least two of the four groups on each carbon are identical. Checking each carbon for four different groups is the whole test.