2-methylprop-1-ene is an isomer of

2-methylprop-1-ene is an isomer of but-2-ene, which is the correct answer (Option C).

To understand this, let's first clarify the meaning of isomers. Isomers are molecules that have the same molecular formula but different structural formulas. In other words, they have the same number of atoms of each element, but their arrangement is different.

Now let's analyze the given molecules:

1. 2-methylprop-1-ene: The parent chain is propene (C₃H₆), and there is a methyl group (CH₃) on the second carbon atom. So, the molecular formula is C₄H₈.

2. But-2-ene: The parent chain is butene (C₄H₈). This molecule has the same molecular formula as 2-methylprop-1-ene (C₄H₈), but the arrangement of atoms is different, so it is an isomer.

The other options have different molecular formulas or arrangements, and therefore, they are not isomers of 2-methylprop-1-ene:

3. 3-methyl but-1-ene: The molecular formula is C₅H₁₀

4. 2-methyl but-1-ene: The molecular formula is C₅H₁₀

5. Pent-2-ene: The molecular formula is C₅H₁₀

So, the correct answer is but-2-ene (Option C).