Choose which part of the given sentence has an error: A. The holy riv…

The correct answer is Option (1) → C

Beside / besides:

• Beside (preposition) means next to, by the side of.
• Besides means in addition to or "apart from".
• This sentence: the river flows by the side of the temple wall, so the preposition is beside.
  Corrected: The holy river Ganga flows beside the wall of the temple.
  Example: She sat beside her grandmother at dinner.

A. The holy river Ganga — No error.

• "The" is correctly used before the name of a river.

B. flows — No error.

• The singular subject "the river" takes the singular verb flows, and the simple present suits a permanent fact.

C. besides the wall — Error.

• "Besides" means "in addition to".
• The sense of next to needs beside.

D. of the temple — No error.

• "Of" correctly links the wall to the temple.