When the reinforcer does not follow every response but occurs after a number of responses have been made or after a passage of time, you are using which reinforcement schedule?

Variable Ratio.

Explanation:

In operant conditioning, a variable-ratio schedule is a reinforcement schedule where the reaction is repeated after an unforeseeable period of reaction. Reinforcement schedule has a key role to play in the working conditioning mechanism. The frequency at which action is repeated will help to decide how easily the response is absorbed and how effective the response may be.