**Core Concept**
Riboflavin, also known as vitamin B2, is an essential nutrient that plays a crucial role in energy production, cellular function, and maintaining healthy skin and mucous membranes. Its deficiency can lead to various clinical manifestations, including lesions on the mouth, tongue, and cornea.

**Why the Correct Answer is Right**
The correct answer to assess riboflavin deficiency is through the measurement of erythrocyte glutathione reductase activity. This enzyme is a flavoprotein that requires riboflavin for its activity. In riboflavin deficiency, the activity of erythrocyte glutathione reductase decreases, making it a reliable indicator of the body's riboflavin status. This test is particularly useful because it reflects the functional status of riboflavin in the body, rather than just its concentration.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because it is not a specific or reliable method for assessing riboflavin deficiency. While riboflavin is essential for energy production, measuring energy expenditure or production is not a direct indicator of riboflavin status.

**Option B:**
This option is incorrect because it is not a specific or sensitive method for assessing riboflavin deficiency. While riboflavin is important for maintaining healthy skin and mucous membranes, clinical examination alone may not be sufficient to diagnose riboflavin deficiency.

**Option C:**
This option is incorrect because it is not a direct or reliable method for assessing riboflavin deficiency. While riboflavin is involved in various biochemical reactions, measuring the activity of a specific enzyme like glucose-6-phosphate dehydrogenase is not a specific indicator of riboflavin status.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that riboflavin deficiency can be challenging to diagnose clinically, and a specific biochemical test, such as erythrocyte glutathione reductase activity, is often required to confirm the diagnosis.

**Correct Answer:** C. Erythrocyte glutathione reductase activity.