Question

what keeps trash from entering a hydraulic reservoir when fresh hydraulic fluid is added?
an inline filter located upstream to the reservoir
the pump
the inlet valve
a strainer or filter in the port

Explanation:

An inline filter located upstream to the reservoir is designed to prevent trash from entering the hydraulic reservoir when fresh fluid is added. Pumps are for moving fluid, inlet valves control fluid entry but not primarily for trash - prevention, and while a strainer or filter in the port can help, an upstream inline filter is more effective in this context.

Answer:

A. An inline filter located upstream to the reservoir