IMO C.

To help preserve ancient Egyptian monuments threatened by high water tables, a Swedish engineering firm has proposed installing pumps, perhaps solar powered, to lower the underground water level and dig trenches around the bases of the stone walls.
(A) to lower the underground water level and dig trenches
installing....digging should be parallel...

lower and dig is incorrect parallelsim.

(B) to lower the underground water level and to dig trenches
installing....digging should be parallel...

lower and dig is incorrect parallelsim.

(C) to lower the underground water level and digging trenches
Correct

(D) that lower the underground water level and that trenches be dug
that trenches is parallel to that lower...wrong parallelism.

(E) that lower the underground water level and trench digging
installing....digging should be parallel...

To help preserve ancient Egyptian monuments threatened by high water tables, a Swedish engineering firm has proposed installing pumps, (perhaps solar powered, to lower the underground water level) and digging trenches around the bases of the stone walls.

hence C

imo B, for parallelism..

swedish firm has suggested 2 things

Installing pumps
Digging trenches

hence C

Ketoag, i think you are seeing only the underlined part.

IMO C

c is correct answer. C follows parallelism properly.

Tricky question I first went with B but ,

To help X …..has proposed 1) installing pumps….. to lower Y and 2) digging trenches