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:

When adding fresh hydraulic fluid to a reservoir, the port used for filling is typically fitted with a strainer or filter to catch trash and contaminants before they enter the reservoir. An inline upstream filter is for fluid in the operating system, not filling. The pump and inlet valve do not serve this filling filtration purpose.

Answer:

D. A strainer or filter in the port