The word, 'unless' means 'except if'. When used in the given blank, it means 'Except if you practice hard, you cannot win the boxing title'. In other words, 'Practicing hard is the only way you can win the boxing title'. As this word meaningfully completes the given sentence, option C is correct.
The word, 'until' means 'up to (the point in time or the event mentioned)'. The word, 'till' means the same as 'until'. When used in the given blank, these words mean: 'Up to the point of time that you practice hard, you cannot win the boxing title'. This sentence does not convey any meaning. Hence, options A and B are incorrect
The word, 'If' means 'in the event that'. It is used to say that a particular thing can or will happen only in the event that something else happens (or becomes true). In other words, it introduces a conditional clause. When used in the given blank, the sentence means 'You cannot win the boxing title in the event that you practice hard'. This sentence conveys a wrong meaning. Hence, option D is correct.