Identifying pentane isomers
Identical compounds are not considered to be isomers. However, the following explanation should clear up any confusion from your question.

A and B are both isopentane and C is n-pentane (normal pentane). These make up two of the three isomers of pentane, the third being neopentane ($\ce{CH3CH(CH3)2CH3}$). The question is referring to pentane as $\ce{C5H12}$, a group of isomers in which n-pentane ($\ce{CH3CH2CH2CH2CH3}$) is included and not identical to.