Which is the right move in a split second decision?

THANKS EVERYONE for your quick responses! Much appreciated! Sounds like it is unanimous that easing off the gas, not accelerating is he safe way to go, and applying the trailer brakes at the same time. My BC is built in to my Expedition and the activation levers are right in front of me so easy to find. This makes a lot of sense and I think it is important to know ahead of time which move to make!
THANKS again!-- this forum is always a big help to me!