Answer:
<h2>The second pyramid, placed in the middle, has a volume of 15 square units.</h2>
Step-by-step explanation:
Givens
- The area of the bases is 15 square units.
- All of them have the same base.
The volum of a pyramid is defined as

Where
is the are of the base and
is the height.
So, all of them have 
The first pyramid has a height of 4, replacing this in the formula we have

The second pyramid has a height of 3

The third pyramid has a height of 12

Therefore, the second pyramid, placed in the middle, has a volume of 15 square units.