**Core Concept**
Carboxylation reactions involve the addition of a carboxyl group (-COOH) to a molecule, often catalyzed by enzymes that require coenzymes to facilitate the reaction. In biochemistry, coenzymes are organic molecules that serve as co-factors for enzymes, enabling them to catalyze specific chemical reactions.

**Why the Correct Answer is Right**
The correct answer is **B. Biotin**. Biotin is a coenzyme for carboxylase enzymes, which catalyze carboxylation reactions. These enzymes, such as acetyl-CoA carboxylase, require biotin to add a carboxyl group to their substrates. Biotin acts as a carrier of carbon dioxide (CO2), facilitating the transfer of the carboxyl group to the substrate.

**Why Each Wrong Option is Incorrect**
* **Option A:** NAD+ is a coenzyme involved in redox reactions, not carboxylation reactions.
* **Option C:** Thiamine pyrophosphate (TPP) is a coenzyme for decarboxylase enzymes, which remove carboxyl groups, not add them.

**Clinical Pearl / High-Yield Fact**
Biotin deficiency can lead to impaired fatty acid synthesis and energy metabolism, resulting in symptoms such as skin rashes, hair loss, and neurological problems.

**Correct Answer:** B. Biotin.