Topic: Oscillatory Motion
Concepts: Equivalence principle, Simple pendulum, Small angle approximation

Solution:

Recall the period of a simple pendulum has the form (by dimensional analysis):

• A) Decreasing $l$ would decrease the period.
• B) Increasing $m$ has no impact on the period.
• C) Increasing $\theta$ would increase $T$ because the small angle approximation $\sin\theta \approx \theta$ breaks down. The actual restoring torque $\tau_{\text{act}} \propto \sin\theta$ is less than the assumed restoring torque $\tau_{\text{SHM}} \propto \theta$ as $\sin\theta<\theta$. This means the actual angular acceleration is less than assumed so it takes longer to return to the equilibrium.
• D) In an elevator accelerating upward, $g_{\text{eff}}>g$ so the period would decrease.
• E) In an elevator moving at constant speed, $g_{\text{eff}}=g$ so the period is unchanged.

Thus, the answer is C.