The general formula for an exponential function is

In this case, b is the base which is any real number greater than 0 and is not 1. Additionally, the domain of an exponential function is all real numbers due to the fact that the exponential function is defined by real numbers. While the range is all real numbers greater than zero.
Answer:
C
Step-by-step explanation:
When completing the square, we essentially want to create a perfect square trinomial by adding a constant.
If we have the following expression:

And we want to complete the square, we will need to divide the b-coefficient by half and then square it.
Thus, the added term should be:

In the given equation, we have:

The b term here is 8. Therefore:

The value we would add would be 16.
The answer is C.
Further notes:
To complete the square, add 16 like mentioned earlier. However, we also need to subtract 16 to balance things out:

The expression inside the parentheses is now a perfect square trinomial. Factor it:

And we are done!