This question is asking us "which number should I multiply by itself so that it gives me 16? "
We can approach the solution in two ways. The first method is guessing the answer and using trial and error. The second method - more neat - is to use mathematics.
Taking the square root of both sides gives
[tex]\sqrt[]{a^2}=\sqrt[]{16}[/tex]The LHS gives a (since the square root of a square is just the number ); therefore, we have
[tex]a=\sqrt[]{16}[/tex]Since,
[tex]\sqrt[]{16}=+4\text{ or -4}[/tex][tex]\begin{gathered} a=+4,\text{ or} \\ a=-4 \end{gathered}[/tex]Hence, the correct answer choice is d. -4
