Under which condition may cargo be carried aft of a divider in an airplane operated under FAR Part 121?

Carrying cargo aft of a divider in an airplane operated under FAR Part 121 is permissible primarily under the condition that it is secured properly. This requirement is essential for maintaining the safety and balance of the aircraft during flight. Properly securing the cargo helps prevent movement that could adversely affect the center of gravity or lead to instability.

In commercial aviation, the management of weight and balance is a critical component of flight safety. Ensuring that carried cargo is secured helps avoid shifts during operations, which can impact aircraft performance and handling characteristics.

The other conditions mentioned, such as weight limitations, crew monitoring, or cargo labeling as dangerous goods, do not specifically address the crucial safety requirement of securing cargo. While these aspects may be relevant in various contexts, they do not supersede the priority of proper securement in accordance with regulatory and safety standards.