The side AB measures option 2.
units long.
Step-by-step explanation:
Step 1:
The coordinates of the given triangle ABC are A (4, 5), B (2, 1), and C (4, 1).
The sides of the triangle are AB, BC, and CA. We need to determine the length of AB.
To calculate the distance between two points, we use the formula 
where (
) are the coordinates of the first point and (
) are the coordinates of the second point.
Step 2:
For A (4, 5) and B (2, 1), (
) = (4, 5) and (
) = (2, 1). Substituting these values in the distance formula, we get

So the side AB measures
units long which is the second option.