Explanation:
The correct answer is D: 3-methylbut-1-ene.

Let's break down the reasoning:

1. 3-methylbut-1-ene:
- The base chain is but-1-ene, which means a 4-carbon chain with a double bond at the first carbon.
- The methyl group (–CH₃) is attached to the third carbon of the butene chain.

This is the correct IUPAC name for the structure described, as it follows the IUPAC nomenclature rules.

Here’s why the others are incorrect:

- 2-methylbut-3-ene: The methyl group is attached to the second carbon in a 4-carbon chain, and the double bond is between the third and fourth carbons.
- 2-methylbut-4-ene: This would imply the double bond is at the fourth carbon, which doesn’t align with standard IUPAC rules.
- 3-methylbut-2-ene: The methyl group is on the third carbon, and the double bond is between the second and third carbons.
- 3-methylpent-1-ene: This indicates a 5-carbon chain with a double bond at the first position, and a methyl group attached to the third carbon, but it doesn’t match the original query.

Thus, the correct IUPAC name is D: 3-methylbut-1-ene.