The correct answer is: but.
The word “but” is used to introduce a contrast or contradiction. In this case, Ajit promised to come, but he did not turn up. This suggests that Ajit was not truthful in his promise, or that something happened to prevent him from coming.
The other options are not as appropriate in this context. The word “and” is used to join two things together, but in this case, Ajit’s promise and his failure to turn up are two opposite things. The word “though” is used to introduce a fact that is surprising or unexpected, but in this case, Ajit’s failure to turn up is not surprising, given that he did not promise to come. The word “still” is used to introduce a fact that is true even though something else is also true, but in this case, Ajit’s failure to turn up does not make his promise any less true.