If both the low-side and the high-side pressures are low, which of these could be the cause? A. High refrigerant charge
B. Defective cooling fan
C. Low refrigerant charge
D. TXV. stuck open

The correct option is;

C. Low refrigerant charge

Explanation:

Where both the low-side and the high-side pressure are low the refrigerant is low which may be as a result of a leak

When there is a low refrigerant charge there will be a larger range over which to compress the refrigerant increasing the work required and thus the heat in the system and the due to insufficient refrigerant, the entire system is under served and the evaporator super-heats due to lack of refrigerant

The liquid-line, the TVX evaporator and compressor will see less fluid flow and hence lesser pressures on the low-side and high-side of the refrigeration system.