No, operants are not necessarily under the control of a stimulus, although they can be made that way through appropriate conditioning techniques. An operant is defined by its effect on the environment. No, positive reinforcers are often pleasant, but not always, and this is not part of the definition. A positive reinforcement is a stimulus which, when applied after a behavior, makes the behavior more frequent or probable in the future. No, positive reinforcement refers to behavior itself, not biological explanations of behavior.
Negative reinforcement is a form of reinforcement.
It increases the frequency or probabilty of a behavior which it follows. No, that is punishment. Unlike negative reinforcement, punishment has the effect of making a behavior less frequent. No, although the two are frequently confused, negative reinforcement is not punishing in its effect. It is reinforcing in its effect.