Correct Answer: has

Solution : The correct choice is the first option.

Explanation:
When dealing with expressions like more than half , half , less than half , etc., they are considered singular entities grammatically. Thus, the correct verb form that agrees with a singular subject is has.

Has is the singular third-person form of the verb, and it correctly matches with half the drain in this sentence to indicate that the desilting action has been completed for more than half of the drain.

Therefore, the complete sentence would be: "More than half the drain has been desilted."