I agree with lachicdina's post above.
Vitamin D is created from exposure of the sun to ultraviolet light (UV). Vitamin D3 is created in the skin when Dehydrocholesterol-7 (Answer A) reacts with ultraviolet light.
To explain dehydrocholesteral a bit further...it is a type of cholesterol in the skin that responds to ultraviolet light when the sun hits the skin. It is then converted into Vitamin D3.
*Fun Facts about this topic: Vitamin D is essential for life in higher animals! It must be supplied in the diet on a regular basis (most of us get it automatically just from our diet and/or being exposed to the sun--however, if you live in say, Alaska or other parts of the world that do not receive adequate sunlight--you may require additional supplements.) Insufficient vitamin D can result in Rickets.
The answer is A. Vitamin D is produced by Dehydrocholestrol, specifically 7-dehydrocholestrol. Then, vitamin D is converted, using 7-dehydrocholestrol, into vitamin D3 in the skin.