Answer:
A. 21 square units
Step-by-step explanation:
Step 1. Make a diagram of the points.
The points are:

Remember that in a point (x, y) the x-value is the first number and the y-value is the second number.
The diagram looks as follows:
Step 2. Connect the points:
Figure ABCD is a rectangle.
Step 3. Find the base and height of the rectangle:
b is the base, and h is the height:

Step 4. Find the area using the following formula:

Substituting the known values:

The result is:

Answer:
A. 21 square units