The correct option is D even though
The clauses in the given sentence "Such a marriage would not be suitable" and "they both are from the same city" are contrast in meaning because being from the same city would be a suitable point for any marriage. Hence a compound conjunction that expresses contrast is used. Compound conjunctions are phrases that perform the role of a conjunction. 'Even though' means 'in spite of' and expresses a contrast in the sentence, like the given one. Hence option D is correct. 'As if' means 'as would be the case if', it expresses a hypothetical/imaginary situation, and does not refer to the contrast in this sentence, hence option A is incorrect. 'As well as' refers to an additional clause and is used when one clause is merely added to another without modifying its meaning, whereas the given sentence has clauses that are opposite in meaning, hence it isn't used here and option B is incorrect. 'As much as' is used in comparison-related sentence, when one clause is compared to another, and both are equal in amount. The given sentence does not have a comparison, but a contrast, hence option C is incorrect.