Which of the following is a restriction for commercial pilots regarding certain aircraft?

The restriction for commercial pilots regarding certain aircraft pertains to their operations involving restricted or limited aircraft. Commercial pilots are not permitted to carry passengers in such aircraft due to the inherent limitations and restrictions placed on these types of vehicles—often related to their intended use or the conditions under which they can be operated.

Restricted aircraft are usually designed for specific purposes, like agricultural or military operations, and may not meet the safety and regulatory standards necessary for transporting passengers. Similarly, limited aircraft are those that have operational limitations that may compromise safety, thus making it unsuitable for standard commercial operations involving passengers.

In contrast, options that suggest commercial pilots can carry passengers in experimental aircraft or in any aircraft without limitations fail to acknowledge the regulatory framework specifically intended to ensure safety. While it's true that certain pilots may operate under specific certifications, those privileges don’t extend to carrying passengers in all classifications, particularly in restricted or limited categories.