The correct option is
C along
NOTE: 'River-banks' means the land at either edge of the river.
Option A - 'On' is used to describe 'above something' that is specific. As 'holy places' and 'riverbanks' are not specific, we can't use 'on' here. Thus option A is incorrect.
Option B - 'At' is usually used for a specific location. For example, 'I am at the top of the world.' As 'river-banks' is not specific, we can't use 'at' here. Thus option B is incorrect.
Option D - 'In' means 'inside'. One can't enter inside a river-bank, hence we can't use 'in' here. Thus option D is incorrect.
Option C - 'Along' means 'from one part of the river to another.' Since the location is unspecifically 'river-banks', we can use 'along' to explain all the parts or portions of the river-banks. Thus option C is the correct answer.
'Most of our holy places, whether on the hills or in the plains, are generally situated along the river-banks...'