Which value must be added to the expression x2 + 16x to make it a perfect-square trinomial? a) 8.b) 32. c) 64. d) 256

Before getting to the actual answer, one must be aware of the way the answer is found. The one and only thing that needs to be done in this case is finding the half of 16 and then squaring it. This obviously gives the result as 64. he correct option among all the options that are given in the question is the third option or option "C". 

Answer
c) 64

Explanation
The general formula of a quadratic equation is 
ax²+bx+c
If the equation is a perfect square, then;

(b/2a)² = c/a

In the equation given, let the number to be added be Y

x² + 16x+Y

(16/(2×1))² = Y/1
Y = 8²
Y = 64