Mani_mba wrote: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
(B) to lower the underground water level and to dig trenches
(C) to lower the underground water level and digging trenches
(D) that lower the underground water level and that trenches be dug
(E) that lower the underground water level and trench digging

The OA is C Please explain.

Always try to simplify complicate sentences to better see how the bits fit together.

Here, the firm is "installing pumps... and dig trenches", which isn't parallel. So, we need to choose (c), "installing pumps... and digging trenches".

IMO C

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.

Remove the red colored portion and try to read the sentence. when a clause have a comma in the front and at the end, it gives extra meaning and we can me remove it from the sentence. The meaning will not be changed.

Here the perhaps solar powered is modifying pumps.

The sentence can be read as

Swedish engineering firm has proposed installing pumps to lower the underground water and digging trenches around the bases.

Cut the comma part to see the actual structure. Typical Parallel structure !

C !