Answer: 4. path paved
Step-by-step explanation:
Based on the options, one can surmise that the underlined area is ''...path and paved it ...''.
After the county cleared the path, they then paved it with packed gravel. These were two distinct actions and so need to be separated when written. Options A through to C manages to do this as both actions are separated by a commas or conjunctions.
In option D however, there was no separation of both actions which makes it grammatically wrong and unacceptable to replace the underlined part.