What is defined by force per unit area

Explanation:

Force per unit area is called Pressure.

Pressure

Explanation:

Let , us understand by a simple example. Let us apply some force into a needle into a cardboard what we observe , we saw that the needle get into the cardboard.

Now we will apply the same force with a water bottle into the same cardboard , now we noticed that it doesn't get buried into it.

In both examples we noticed that force applied in both the cases is equal but the surface area of the object in which force applied is different.

Here come the term pressure.

Pressure is defined as force applied per unit area.

Therefore , as area increases pressure also decreases . That's why bottle don't get buried but needle go through it.  

the answer is a. hope this .