**Core Concept**
Vitamins are essential nutrients that play crucial roles in various bodily functions, including immune system modulation. Some vitamins have been found to exhibit anti-infective properties, helping to combat infections and diseases.

**Why the Correct Answer is Right**
Vitamin C (Ascorbic acid) is a crucial water-soluble vitamin that has been extensively studied for its anti-infective properties. It acts by enhancing the production of white blood cells, such as neutrophils and macrophages, which play a vital role in the body's defense against infections. Additionally, Vitamin C has been shown to have antioxidant properties, which help protect against oxidative stress caused by pathogens. Its ability to scavenge free radicals and enhance the activity of immune cells makes it an effective anti-infective agent.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while Vitamin A is essential for maintaining a healthy immune system, it does not directly exhibit anti-infective properties like Vitamin C.
**Option B:** This option is incorrect because Vitamin K is primarily involved in blood clotting and does not have significant anti-infective effects.
**Option C:** This option is incorrect because Vitamin E is an antioxidant, but it does not have the same level of anti-infective activity as Vitamin C.

**Clinical Pearl / High-Yield Fact**
Vitamin C deficiency can lead to impaired immune function, making individuals more susceptible to infections. This is a critical concept for medical students to remember, as Vitamin C supplementation is often recommended for patients with compromised immune systems.

**Correct Answer: C. Vitamin C (Ascorbic acid)**