The correct option is C yet
The given sentence expresses contrast, as suggested by the word 'although'.
'Although' is often followed by 'yet'; the latter also expresses contrast. 'Although' means 'but, in spite of' and 'yet' means 'still, even then'.
Hence, option C is correct.
'But' also refers to contrast, but it means the same as 'although'. Hence, the meaning is not complete. The sentence means that 'even though Mohit lost the match, still he is happy.'
So, option A is incorrect.
'And' merely adds or connects two clauses that do not affect one another. The clauses here contrast each other, hence is incorrect.
'Then' is generally not used as a conjunction.