Which shows an isomer of the molecule below?

Above question is incomplete. Complete question is attached below
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Correct Option D i.e. CH3CH2CH2CH2CH3

Reason:
Isomers are the compounds having same molecular formula, but different structural formula.

The molecular formula of the compound is C5H12

The compound present is question (i.e. 2-methyl butane) and option D (n-pentane) has same molecular formula. Hence, they are isomers.

However, compound present in option A, is the same compound provided in question. Hence, it is not an isomer. Compounds reported in option B and C has one carbon atom less and more, respectively as compared to the compound reported in question. Hence, they are also not isomers. 

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