Charging by occurs when electrons are transferred from one object to another by direct contact
a. conduction
b. induction
c. reduction
d. friction

Charging by Conduction occurs when electrons are transferred from one object to another by direct contact.

Explanation:

Charging by Conduction happens when a charged object and a neutral object comes in contact. This is why it is also called charging by contact.

How it happens?

We know that like charges repel each other. Let us consider a negatively charged ball. It must have excess electrons and these electrons are as far from each other as they can get. When this negatively charged get in contact with a neutral ball. The electrons rush to the neutral ball to get away from the electrons on the negatively charged ball. In this act of getting away of electrons from each other, the neutral ball gets charged too.

a.) travels faster in solids because the particles are closer together.

air is a gas water is a liquid