Is the Vitamin D that our skin makes when exposed
to the sun's rays natural or synthetic?

The Vitamin D that is made when exposed to sun rays are natural. Your skin naturally intakes the sunlight as Vitamin D.

Explanation:

This happens naturally for everyone, although some people are able to make less Vitamin D than others.

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current