Identify the numbers in the grid and express them as powers of a common base, if possible.

The given numbers are 32, 8, and 128.
We can express these numbers as powers of 2:
$32 = 2^5$
$8 = 2^3$
$128 = 2^7$

Let the missing number be $X$. The grid can be represented by its exponents:

Consider the sum of the exponents in the first row:
$5 + 3 = 8$

Now, assume the same pattern applies to the second row, meaning the sum of the exponents in the second row is also 8.
Let the exponent of the missing number be $x$.
$7 + x = 8$

$x = 8 - 7$
$x = 1$

Since $x = 1$, the missing number is $2^1$.
$X = 2^1 = 2$

The calculated missing number is 2.
The options are:
A. 4
B. 3
C. 6
D. 2
E. 8

The calculated value matches option D.

The final answer is $\boxed{\text{D. 2}}$.