Option A - 'More economical' is the comparative degree of the adjective. The given sentence doesn't compare anything, hence we can't use 'very precious' which is the comparative of an adjective here. Thus option A is incorrect.
Option B - 'Economics' is a noun, which means 'the study of trade'. As we need an adjective, we can't use a noun here. Thus option B is incorrect.
Option D - 'Most economical' is the superlative form of the adjective, which can only be used to talk about the superior degree. As the sentence already uses the superlative form of adjective 'most Indians', we can't use the superlative form again. Thus option D is incorrect.
Option C - 'Economical' is an adjective, which means 'not using a lot of money'. As the given sentence is a general statement, we can use the positive degree of adjective here. Thus option C is the correct answer.
'Most Indians are economical with their domestic expenditure.'