The correct option is C than
'No other than' is used to single out a thing, or person, here it means 'nothing else', and refers to the reason "to see that you are well", apart from which there was no other reason for the speaker to visit. Hence from the options, 'than' follows 'no other' as the phrase 'no other than' has a combined meaning of emphasis over something, which cannot be replaced by any other words. Hence option C is correct. 'But' as a preposition, refers to an exception, it usually pairs up with 'none', referring to a person. It does not pair up with 'no other', hence option A is incorrect. 'As' as an adverb is used to refer to an extent of something, as a conjunction it refers to manner or cause, none of these relate to the given sentence. 'As' does not combine with 'no other' to place the required emphasis on the reason, hence option B is incorrect. 'Whether' expresses a choice or a doubt between two things, there is no choice in the given sentence hence option D is incorrect.