Option 1: Brake.
What will happen? More than likely if you brake, you will spin. All driving, whether you think about it or not is about controlling the contact patch. Weight in a vehicle is somewhat fluid, and when you accelerate or decelerate the weight shifts. In a corner, if you start to slide and you tap the brakes, the weight shifts forward and physics kicks in. No car is smart enough to fix that even with VSC (vehicle stability control). Straight-line braking, on the other hand. is much more simple, and it has to do with just modulating tire spin. So if in a corner and the rear slides out, don't apply the brake immediately.
Option 2: Let off the gas.
See Option 1. This is the same scenario only a bit milder. It would be better than jabbing the brakes, but more than likely will still induce the car to continue to slide and eventually spin.
Option 3: Steer into it and slowly decrease throttle.
Remember the movie cars when Doc is trying to teach Lightning how to drift in the dirt? That was pretty accurate; you have to control the slide by steering into it and holding on. (I have three boys ages 3-7 so that movie is burned into my mind.)
Most people find this a bit counter-intuitive. Honestly, I started learning this craft as a young driver with a 1979 Camaro in snow-covered parking lots with my dad. I have had the opportunity to do a few driving schools and drive a SKID car, and it was surely tricky but helpful. All emergency maneuvers, just like performance maneuvers, take your car to the limit and controlling it in both situations requires the same skills. If your local law enforcement allows it, try it out carefully in a large parking lot near home and get a feel for how steering into a corner actually helps keep control!
If you can't find a safe place to do this, give the Mid-Ohio Driving Schoool a try. If you have a teen, enroll them in a Tire Rack Street Survival school and give them great experience in their own cars learning to control the vehicle at the limit and learn emergency maneuvers!