Final answer:
A half-off gym membership for attending 85% of group fitness classes for 3 months is an example of a material reinforcer.
Step-by-step explanation:
The given scenario of offering a half-off gym membership when a participant attends 85% of group fitness classes for 3 months is an example of a material reinforcer. Material reinforcers are incentives provided to individuals in the form of physical or tangible rewards, such as discounts, gifts, or other benefits. In this case, the half-off gym membership serves as a material reinforcer to encourage attendance and participation in group fitness classes.