What is one type of connector allowed between trenches in a serial distribution system?

A drop box is specifically designed to control the flow of effluent within a serial distribution system, allowing for even distribution across trenches while maintaining optimal operation. It serves as a junction that connects different trenches and manages the hydraulic gradients, ensuring that each trench receives the appropriate volume of effluent.

In contrast, other options such as flexible hoses or stub pipes may not provide the necessary structural support or flow control required in such an application. Standard elbows are generally used for redirection of piping but do not facilitate distribution in the manner that a drop box does. Therefore, the drop box is the correct choice as it is integral to effectively managing the distribution of wastewater in a septic system.