Explanation

Vitamin-D is a sterol derivative, which is synthesised from cholesterol. Vitamin-$D_{2}$ is formed from plant sterol-ergosterol, which occurs in yeast and some other fungi, in the presence of UV light. Vitamin- $D_{3}$ is synthesised from animal sterol-7 hydroxy cholesterol, which occurs in animals. Its synthesis also requires UV light. Deficiency of vitamin-D causes type I and type II diabetes in humans