The period of a swing, also known as a pendulum, depends on its length, that is, the distance from the pivot point to the center of mass and gravitational acceleration, but not on the mass swinging. When a girl stands up, the distance from the swing`s pivot point to her center of mass decreases, effectively making the `pendulum` shorter. According to the formula of the period for simple pendulum T=2π√(L/g) where L is the length of the pendulum, g is the acceleration due to gravity, and T is the time period, a shorter pendulum results in a shorter period, meaning the swing will go back and forth more quickly. Therefore, the correct answer is option 1 "be shorter". Options 2 and 3 are incorrect because the period will not be longer nor does it depends on the height of the girl. Option 4 is also incorrect because the period will indeed change when the girl changes her position on the swing.