What is the length of the missing side? a. 16 mm b. 28 mm c. 42 mm d. 69.5 mm

Discussion
It certainly cannot be D. That answer is reserved for adding two sides to get the hypotenuse. The hypotenuse is already given (53), so 69.5 is impossible.

43 might be right
28 might also be right.
16 might be right as well.

So you have to calculate to get the answer. 
c = 53
b = 45
a = ?

a^2 + b^2 = c^2             Substitute into the Pythagorean Theorem
a^2 +45^2 = 53^2          Expand
a^2 + 2025 = 2809        Subtract 2025 from both sides 
a^2 = 2809 - 2025
a^2 = 784                      Take the square root of both sides.
sqrt(a^2) = sqrt(784)        
a = 28 <<<< Answer