Ergocalciferol is name of vitamin:

Ergocalciferol is name of vitamin $D$. 

This vitamin is essential for absorption of calcium and phosporus in the body.

 It is used to treat and prevent bone disorders (such as rickets, osteomalacia). Vitamin D is made by the body when skin is exposed to sunlight. 

Ergocalciferol is name given to vitamin $D_2$.

Option D is correct.