Which of the following changes will result in an [i]increase[/i] in the period of a simple pendulum?

(A) Decrease the length of the pendulum
(B) Increase the mass of the pendulum
(C) Increase the amplitude of the pendulum swing
(D) Operate the pendulum in an elevator that is accelerating upward
(E) Operate the pendulum in an elevator that is moving downward at constant speed.
To determine which change will result in an increase in the period of a simple pendulum, we need to analyze the factors that affect the period of a simple pendulum. The period \( T \) of a simple pendulum is given by the formula:

\[ T = 2\pi \sqrt{\frac{L}{g}} \]

where:
- \( T \) is the period of the pendulum,
- \( L \) is the length of the pendulum,
- \( g \) is the acceleration due to gravity.

Let's analyze each option:

1. **Decrease the length of the pendulum (Option A):**
   \[
   T = 2\pi \sqrt{\frac{L}{g}}
   \]
   If \( L \) decreases, the value inside the square root decreases, which means \( T \) decreases. Therefore, decreasing the length of the pendulum will decrease the period.

2. **Increase the mass of the pendulum (Option B):**
   The period \( T \) of a simple pendulum is independent of the mass of the pendulum. Therefore, increasing the mass will not affect the period.

3. **Increase the amplitude of the pendulum swing (Option C):**
   For small angles (small amplitude), the period \( T \) is approximately given by \( T = 2\pi \sqrt{\frac{L}{g}} \). However, for larger amplitudes, the period increases slightly because the approximation \( \sin \theta \approx \theta \) is no longer valid. Therefore, increasing the amplitude of the pendulum swing will result in an increase in the period.

4. **Operate the pendulum in an elevator that is accelerating upward (Option D):**
   When the elevator is accelerating upward, the effective acceleration due to gravity increases to \( g_{\text{eff}} = g + a \), where \( a \) is the acceleration of the elevator. The period \( T \) becomes:
   \[
   T = 2\pi \sqrt{\frac{L}{g + a}}
   \]
   Since \( g + a \) is greater than \( g \), the value inside the square root decreases, which means \( T \) decreases. Therefore, operating the pendulum in an elevator that is accelerating upward will decrease the period.

5. **Operate the pendulum in an elevator that is moving downward at constant speed (Option E):**
   When the elevator is moving downward at constant speed, there is no change in the effective acceleration due to gravity. Therefore, the period \( T \) remains the same. Operating the pendulum in an elevator that is moving downward at constant speed will not affect the period.

Conclusion:
The only option that results in an increase in the period of the pendulum is increasing the amplitude of the pendulum swing.

The final answer is \(\boxed{C}\)