The "C" option stands for Clear. When the trigger man, 2 in this example, calls this from the triangle position then 5 and 1 immediately and quickly go to the other side of the floor. They "clear-out" to the opposite side leaving 2 to take his man one-on-one to the basket. (Remember, we must be good actors in basketball so 5 might act like he's screening for 4, and 4 might act like he's screening for 1, who could act like he's coming off the screen from 4 - you don't want to just turn and watch 2 with the ball because the defense will then quickly figure out the play).
If a shot is taken, 1, 4 and 5 CRASH the boards while 3 stays at the point for defensive protection.
("C" can also be called out by the point guard. In this case if the point guard passes to the wing then he cuts through to the basket and continues out to the opposite corner area. If the point guard dribbles to the wing then the wing man clears out to the opposite corner area.)

2 takes his man one-on-one trying to get all the way to the basket.
If he can't beat his man then the triangle is reset, either on the same side or on the opposite side.
(Another option may be, instead of calling out "C" every time for this play would be to start the clear out play by calling out any color.)