The correct answer is option 4: Vitamin D.

Vitamin D is unique because it can be synthesized by the human body. When our skin is exposed to sunlight, it produces a form of cholesterol called 7-dehydrocholesterol, which can be converted into vitamin D. This process occurs in the presence of UVB rays.

However, it`s important to note that not everyone is able to produce adequate amounts of vitamin D through sunlight alone. Factors such as skin pigmentation, geographic location, time of year, and use of sunscreen can all affect the skin`s ability to produce vitamin D. Therefore, it is recommended to also obtain vitamin D from dietary sources such as fatty fish, fortified dairy products, and supplements to ensure adequate intake.

The other options provided are incorrect:

- Option 1: Vitamin Bj is not a known vitamin.

- Option 2: Vitamin B12 is an essential vitamin that plays a critical role in nerve function and red blood cell production. However, it cannot be synthesized by the human body and must be obtained from dietary sources such as meat, fish, eggs, and dairy products.

- Option 3: Vitamin C, also known as ascorbic acid, is another essential vitamin that is not synthesized by