C

In a simple pendulum, the period of oscillation $T$ can be described by the formula:
$$T = 2\pi \sqrt{\frac{L}{g}}$$
where $T$ is the period, $L$ is the length of the pendulum, and $g$ is the acceleration due to gravity.
Now, let's analyze each statement:

Option A: Directly proportional to square root of the length of the pendulum.
This is true as $T \propto \sqrt{L}$.

Option B: Inversely proportional to the square root of the acceleration due to gravity.
This is also true as $T \propto \frac{1}{\sqrt{g}}$.

Option C: Dependent on the mass, size, and material of the bob.
This statement is not true as the mass and material of the bob do not affect the period of a simple pendulum (for small amplitudes).

Option D: Independent of the amplitude.
This is true for small amplitudes, where the small angle approximation holds.

Therefore, the statement that is not true is Option C.