You must remember that a polynomial is written like so...
ax^2 + bx + c
In this case...
a = 1
b = 16
c = 64
To factor you must find two numbers who both add up to b (16) AND multiply to c (64)
8 + 8 = 16
8 * 8 = 64
so...
C) (x + 8)(x + 8)
Hope this helped!
~Just a girl in love with Shawn Mendes