In the given grid, find the missing number: [32, 8; 128, ?]

The numbers in the grid are 32, 8, and 128. These can all be expressed as powers of 2.

$$32 = 2^5 \\ 8 = 2^3 \\ 128 = 2^7$$

Let the missing number be $X$. We assume $X$ is also a power of 2, so $X = 2^x$.

The grid of numbers is:

$$\begin{bmatrix} 32 & 8 \\ 128 & ? \end{bmatrix}$$

Replacing the numbers with their exponents (base 2):

$$\begin{bmatrix} 5 & 3 \\ 7 & x \end{bmatrix}$$

Observe the sum of the exponents in the first row:

$$5 + 3 = 8$$

Assume the same pattern applies to the second row, meaning the sum of the exponents in the second row is also 8.

$$7 + x = 8$$

Since $X = 2^x$ and $x = 1$:

$$X = 2^1 \\ X = 2$$

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 $\boxed{\text{D. 2}}$.