A girl is swinging in a swing in a sitting position. How will the period of the swing be affected if she stands up?

Let the swinging be a simple pendulum,

So the time period of the simple pendulum is:

$T=2\pi \sqrt{\frac{L}{g}}$

Where L is the length,

Since length of the simple pendulum is equal to the distance from the point of suspension to CG of the oscellting body,

When the girl stands up the distance of CG from the point of suspension will decrease.

Therefore time period of the oscillation also decreases.