The Pythagorean Theorem can be used to find the missing side length of a triangle when two other side lengths are known.

C) 35.2

To find the missing side length, b, we can use the modified version of the Pythagorean Theorem, which states c = √(a^2 + b^2).

In this case, we have:
a = 20
c = 29

Substituting these values into the formula c = √(a^2 + b^2), we get:
29 = √(20^2 + b^2)

To isolate b^2, we can square both sides of the equation:
29^2 = (20^2 + b^2)^2
841 = 400 + b^2

Subtracting 400 from both sides of the equation, we have:
441 = b^2

To find the value of b, we take the square root of both sides:
b = √441
b = 21

Therefore, the value of b is D) 21.

To find the missing side length, b, using the Pythagorean Theorem, we can rearrange the formula as follows:

b = √(c^2 - a^2)

Given that a = 20 and c = 29, we can substitute these values into the formula:

b = √(29^2 - 20^2)

Calculating this expression:

b = √(841 - 400)

b = √441

Therefore, the value of b is the square root of 441, which is 21.

So, the correct answer is:

D) 21