Final answer:
In the context of retirement plans, such as social insurance programs like Social Security and Medicare, service impacts eligibility, accrual of benefits, and vesting, but not contributions, which are usually a fixed percentage of income.
Step-by-step explanation:
The question is focused on the basis upon which a retirement plan determines various aspects of its operation. Service is a key component in such plans, particularly in relation to Social Security and Medicare, which are forms of social insurance. These programs involve workers making regular contributions through income deductions. However, not all aspects mentioned in the question are dictated by service. Specifically, the one aspect that service does not determine in this context is d: contributions. Eligibility, accrual of benefits, and vesting may all be influenced by the length and nature of service, but contributions are typically determined as a fixed percentage of income or a set amount as dictated by the plan or regulatory requirements, rather than the service itself.