**Core Concept**
Vitamin C, also known as ascorbic acid, is an essential nutrient with various physiological functions, including antioxidant properties, collagen synthesis, and iron absorption. The richest sources of vitamin C are typically fruits and vegetables that are rich in ascorbic acid.

**Why the Correct Answer is Right**
The correct answer is a citrus fruit, which is known for its high content of ascorbic acid. Citrus fruits, such as oranges, lemons, and grapefruits, are among the richest sources of vitamin C, with oranges being the highest. The ascorbic acid in citrus fruits is responsible for their characteristic sour taste and plays a crucial role in their antioxidant properties.

**Why Each Wrong Option is Incorrect**
**Option A:** Tomatoes are often thought to be a rich source of vitamin C, but their ascorbic acid content is relatively low compared to citrus fruits.
**Option B:** Bell peppers are a good source of vitamin C, but their content is significantly lower than that of citrus fruits.
**Option C:** Potatoes are a starchy vegetable that contains some vitamin C, but not enough to be considered a rich source.

**Clinical Pearl / High-Yield Fact**
Remember the "CITRUS" acronym to recall the richest sources of vitamin C: CITRUS fruits, including oranges, lemons, and grapefruits.

**Correct Answer:** C. Oranges