**Core Concept**
Vitamin C is an essential water-soluble vitamin, crucial for collagen synthesis, iron absorption, and immune function. It is abundant in fresh fruits, vegetables, and certain animal products. The question is testing the knowledge of food sources rich in vitamin C.

**Why the Correct Answer is Right**
Option C, egg yolk, is the poorest source of vitamin C among the given options. This is because vitamin C is a thermolabile vitamin that is easily destroyed by heat and oxygen during cooking. Egg yolks, being high in fat and often cooked at high temperatures, have minimal vitamin C content. In contrast, citrus fruits, strawberries, and bell peppers are rich in vitamin C.

**Why Each Wrong Option is Incorrect**
**Option A:** None of the options are listed, but typically, citrus fruits like oranges, lemons, and limes are excellent sources of vitamin C.
**Option B:** Strawberries are a good source of vitamin C, although not as rich as citrus fruits.
**Option D:** Bell peppers are a good source of vitamin C, especially the green ones.

**Clinical Pearl / High-Yield Fact**
Fresh fruits and vegetables are rich in vitamin C, making them essential components of a balanced diet. Cooking and processing can destroy vitamin C, so it's best to consume these foods raw or lightly cooked.

**Correct Answer:** C. Egg yolk.