An alkane which can exhibit optical activity is:

Optical activity requires a chiral center (a carbon with four different substituents). 3-Methylhexane has a chiral center at carbon 3, with four different groups: H, methyl, ethyl, and propyl. 3-Methylpentane has two identical ethyl groups at the chiral center, so it is achiral. Isopentane and neopentane lack chiral centers. Thus only 3-methylhexane can exhibit optical activity.