What does an officer swinging a yellow light indicate?

A)That users must moderate their speed.
B)It requires the road users towards whom the officer directs the light to stop.Correct
When an officer swings a yellow or red light, it requires road users towards whom the officer directs the light to stop.
Do not confuse this with when the officer moves an extended arm alternately up and down, which requires drivers approaching the officer on the side corresponding to the arm performing the signal and perpendicular to that arm to reduce the speed of their vehicle.
Do not confuse this with when the officer moves an extended arm alternately up and down, which requires drivers approaching the officer on the side corresponding to the arm performing the signal and perpendicular to that arm to reduce the speed of their vehicle.