Determine which set of side measurements could be used to form a right triangle.

To determine which set of side measurements could be used to form a right triangle, we can apply the Pythagorean theorem.

The theorem states that in a right triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides.

Let's check each set of side measurements:

a) In this case, 4^2 + 11^2 = 16 + 121 = 137, and 20^2 = 400. As 137 is not equal to 400, set (a) cannot form a right triangle.

b) Here, 16^2 + 21^2 = 256 + 441 = 697, and 25^2 = 625. Again, 697 is not equal to 625, so set (b) cannot form a right triangle.

c) Now, 5^2 + 13^2 = 25 + 169 = 194, and 25^2 = 625. Once more, 194 is not equal to 625, so set (c) cannot form a right triangle.

d) In this case, 3^2 + 4^2 = 9 + 16 = 25, which is equal to 25^2 = 625. Finally, set (d) satisfies the Pythagorean theorem and can form a right triangle.

Therefore, the set of side measurements that could be used to form a right triangle is d) 3, 4, 5.