Which of the following is always true when a substance undergoes a physical change ?
a. the composition changes.
b. a change in color occurs.
c. no new substance is formed.
d. its specific gravity remains the same.

c. no new substance is formed.

Explanation:

A physical change is defined as a change in which there is alteration in shape, size etc. No new substance gets formed in these reactions.

Example: Melting of ice

A chemical change is defined as a change in which a change in chemical composition takes place. A new substance is formed in these reactions.  Chemical reactions are usually characterized by change in color, heat change, precipitate formation and evolution of bubbles.

Example: Oxidation of magnesium

atomic number of carbon is 6

electronic configuration of carbon is 1s²2s²2p²

orbital diagram of carbon is as shown here in the image.