The main drawback(s) of underground system over overhead system is/are-?

it is B. Heavy initial costing like Excavation and backfill. 

HEAVY INITIAL COAST(COAST OF DIGGING ,MATERIALS SUCH AS CABLES COST MORE THAN OVERHEAD LINES),ANOTHER DRAWBACK IS MAINTENANCE

Answer is: option (b) Heavy initial cost

 D. Inductive interference between power and communication circuit

it is difficult to fixfault place in under ground and the interference with communication line,exposure to lightining in over head

This question can be accurtely answered depending on engineering and economical constraints. The limitations includes, but not limited to, the folllowing:

- Cost: Even though the installation of transmission lines are not really low in price, underground systems require more digging and installation work. On top of that, maintenace cost is lower for overhead lines, because of the ease of acccess. For these two reasons, in terms of cost (initial investment) and maintenace, overhead lines are more cost-effective.

- Lines seperations: Underground lines are closely seperated, which make the conductors act as stronger capacitors. As a result, underground lines have higher capacitance, which affects the the Ferranti effect (higher receiving end than sending end). Plus, with higher capacitance, more charging current is drawn. And for that reason, underground lines have length limitations.

- Safety: Overhead lines are supported by long towers, which obstacle flying objects, birds, and even vehicles. However, underground systems are more isolated and less problamatic with public activities.

- Insulation: Unlike underground lines, overhead lines have a better heat dissipation surrounding, which allow them to operate in high voltage.

- Faults and interrupts: Overhead lines are exposed to lightings, Corona, and many other exterior factors that cause damages as well. However, undeground lines are more isolated and protected.