The correct answer is a time of the day, latitude and season.
There are numerous factors that minimize the production of skin's Vitamin D3, including aging, increased pigmentation of the skin, and the topical application of a sunscreen.
A change in the zenith angle of the sun resulting due to a change in a season of the year, latitude, or time of the day drastically affects the production of skin's Vitamin D3. So, it is tough to determine the levels of vitamin D3 on the basis of the exposure of the sun.