Answer:
variable ratio
Step-by-step explanation:
Variable ratio: The term has been widely used in operant conditioning.
A variable-ratio schedule is a kind of reinforcement schedule, in which a particular response is being reinforced after an unforeseeable number of responses. It creates a high and steady response to the desired behavior and the extinction of this behavior is very resistant.
Example: Lottery games and gambling, etc.
The example given in the question above is an example of a variable-ratio schedule of reinforcement.