**Core Concept**
Oxidation of lactate to pyruvate is a critical step in cellular respiration, involving the conversion of lactate back into pyruvate through the action of lactate dehydrogenase. This process is essential for the regeneration of NAD+ from NADH, allowing glycolysis to continue.

**Why the Correct Answer is Right**
The correct answer is **B. NAD+** because the oxidation of lactate to pyruvate requires the coenzyme NAD+, which acts as an electron acceptor in this reaction. Lactate dehydrogenase catalyzes the conversion of lactate to pyruvate, reducing NAD+ to NADH in the process. The regeneration of NAD+ is crucial for the continuation of glycolysis and subsequent cellular respiration.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because thiamine (vitamin B1) is involved in the decarboxylation of alpha-keto acids in the pyruvate dehydrogenase complex, but not in the oxidation of lactate to pyruvate.
**Option C:** This option is incorrect because riboflavin (vitamin B2) is involved in energy production, but primarily through its role in the electron transport chain, not in the oxidation of lactate to pyruvate.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the oxidation of lactate to pyruvate is a critical step in cellular respiration, and any disruption to this process can lead to lactic acidosis, a condition characterized by an excessive accumulation of lactate in the blood.

**Correct Answer: B. NAD+**