The correct answer is A. 440 V.
The underground system cannot be operated above 440 V because of the risk of fire and explosion. The higher the voltage, the greater the risk of arcing and sparking, which can ignite flammable materials. In addition, higher voltages can cause more damage to electrical equipment if there is a fault.
Option B, 11 V, is too low to be practical for most applications. Option C, 33 V, is a common voltage for underground systems, but it is still possible for arcing and sparking to occur at this voltage. Option D, 66 V, is a higher voltage that is more likely to cause arcing and sparking, and therefore it is not recommended for underground systems.
It is important to note that the voltage at which an underground system can be operated safely will vary depending on the specific conditions of the installation. For example, the type of soil, the presence of flammable materials, and the proximity of other electrical equipment will all need to be considered. It is always best to consult with a qualified electrician to determine the safe operating voltage for a particular underground system.