What is the missing number in the grid: [[16, 9, 4], [25, 16, 9], [36, 25, ?]]?

The numbers in the grid are:

Notice that all the visible numbers are perfect squares.

Let's find the square roots for each cell:

Looking at the square roots:

• In the first row, the numbers are $4, 3, 2$ (decreasing by 1).
• In the second row, the numbers are $5, 4, 3$ (decreasing by 1).
• This pattern suggests that in each row, the numbers decrease by 1 from left to right.

Following the pattern for the third row, the numbers should be $6, 5, (5-1)$.
So, the missing square root is $5-1=4$.

The missing number is the square of the missing square root:

The calculated missing number is 16.
The options are: a. 49, b. 25, c. 9, d. 16.
The correct option is d.

The final answer is $\boxed{\text{d. 16}}$.