Option 1: Vitamin A is important for maintaining good vision, healthy skin, and promoting the immune system`s function. While it does support the immune system, it is not specifically known for preventing infections.

Option 2: The vitamin B complex includes multiple vitamins that play a role in energy production, nerve function, and supporting metabolism. While they are necessary for overall health, they do not have a direct impact on preventing infections.

Option 3: Vitamin C, also known as ascorbic acid, is an essential vitamin that is well-known for its immune-boosting properties. It helps to stimulate the production of white blood cells, which are important for fighting off infections. Vitamin C is also a powerful antioxidant, which helps to protect the body from harmful free radicals.

Option 4: Vitamin D is crucial for bone health and regulating calcium levels in the body. Although it does play a role in immune function, it is not as directly involved in preventing infections compared to vitamin C.

Therefore, the correct answer is option 3, vitamin C. It is a key vitamin for boosting the immune system and helping to prevent infections.