Which of the following is a feature of supplemental term riders?

Supplemental term riders are additional features that can be attached to a permanent life insurance policy, allowing policyholders to increase their coverage temporarily. This is especially beneficial for individuals who may need additional coverage during specific periods of life, such as raising children or during periods of high financial obligation.

By adding a supplemental term rider to a permanent policy, the insured can secure an increased death benefit for a predetermined term without altering the original permanent policy. This flexibility allows for tailored insurance solutions based on the policyholder's changing needs.

The other options do not accurately describe supplemental term riders. They do not extend coverage beyond the primary policy’s term, they do not need to be purchased separately, nor do they limit the death benefit of the primary policy. Rather, they enhance the coverage during a specific time frame while the base permanent policy remains intact.