1. A swing behaves approximately like a pendulum.

2. The time period of a pendulum is $T = 2\pi\sqrt{\frac{L}{g}}$ where:

   • $L$ = effective length of the pendulum
   • $g$ = acceleration due to gravity

3. For a child on a swing, the effective length $L$ is the distance from the point of suspension to the centre of mass of the child-swing system.

4. When the child is in sitting position, the centre of mass is relatively lower.

5. When the child stands up, the centre of mass moves upward, closer to the point of suspension. Therefore, the effective length $L$ decreases.

6. Since $T \propto \sqrt{L},$ a decrease in $L$ causes the time period $T$ to decrease.

7. Hence, the correct option is: $\boxed{\text{B: decrease}}$

Verification with stored answer

Stored correct answer: B

Our derived answer is also B, so they agree.