Return lines in a non-pressurized hydraulic system reservoir should enter the tank well below the fluid surface level to __________.

Correct Answer: Option C — prevent foaming

Return lines must terminate below the fluid level to prevent splashing and aeration. If the return fluid falls through the air space, it entrains air, causing foaming. Foaming reduces oil bulk modulus, leads to erratic actuator operation, and promotes cavitation, which is detrimental to hydraulic pump longevity.