Observe the numbers in each cell:

Observe the numbers in each cell:
Row 1: 16, 9, 4
Row 2: 25, 16, 9
Row 3: 36, 25, ?

Let's look at the square roots of the numbers in each row:
For Row 1:
$\sqrt{16} = 4$
$\sqrt{9} = 3$
$\sqrt{4} = 2$
The sequence of square roots is $4, 3, 2$.

For Row 2:
$\sqrt{25} = 5$
$\sqrt{16} = 4$
$\sqrt{9} = 3$
The sequence of square roots is $5, 4, 3$.

In both Row 1 and Row 2, the square roots of the numbers form a decreasing sequence by 1.
The first number's square root is 1 greater than the second, and the second is 1 greater than the third.

For Row 3:
$\sqrt{36} = 6$
$\sqrt{25} = 5$
Following the pattern, the square root of the missing number should be 1 less than 5.
So, the square root of the missing number is $5 - 1 = 4$.

If the square root of the missing number is 4, then the missing number is $4^2$.

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 $\boxed{\text{d. 16}}$