Final answer:
d. Younger siblings of the policyholder
Riders can be added to life insurance policies to cover stepchildren, legally adopted children, and biological children. However, they typically do not include younger siblings of the policyholder.
Step-by-step explanation:
Riders covering additional insureds can be added to life insurance policies to extend coverage to members associated with the policyholder. However, not all individuals can be covered under such riders.
Specifically, riders typically cover stepchildren, legally adopted children, and biological children. On the other hand, they generally do not cover younger siblings of the policyholder, unless there are exceptional provisions within the policy.