**Core Concept**
The conversion of a fatty acid to its active form involves the process of activation, where a fatty acid is converted into its coenzyme A (CoA) derivative. This process is crucial for the subsequent steps of fatty acid metabolism, including beta-oxidation and fatty acid synthesis. Acyl-CoA synthetase plays a key role in this process by catalyzing the activation of fatty acids.

**Why the Correct Answer is Right**
Acyl-CoA synthetase, also known as fatty acyl-CoA synthetase, is a family of enzymes that catalyze the conversion of a fatty acid into its CoA derivative. This process involves the transfer of a CoA molecule to the carboxyl group of the fatty acid, resulting in the formation of acyl-CoA. The enzyme uses ATP to drive this reaction, which is essential for the subsequent steps of fatty acid metabolism. The correct answer is Acyl-CoA synthetase because it is the enzyme responsible for the activation of fatty acids.

**Why Each Wrong Option is Incorrect**
**Option A:** Enoyl CoA hydratase is an enzyme involved in the beta-oxidation of fatty acids, but it is not responsible for the activation of fatty acids.
**Option B:** Thiolase is another enzyme involved in beta-oxidation, specifically in the breakdown of acyl-CoA molecules.
**Option C:** Acyl-CoA dehydrogenase is also involved in beta-oxidation, specifically in the conversion of acyl-CoA to enoyl-CoA.

**Clinical Pearl / High-Yield Fact**
The activation of fatty acids by Acyl-CoA synthetase is a critical step in fatty acid metabolism, and defects in this enzyme can lead to fatty acid oxidation disorders, such as medium-chain acyl-CoA dehydrogenase (MCAD) deficiency.

**Correct Answer:**
✓ Correct Answer: A. Acyl-CoA synthetase