The correct option is D else
The sentence is a compound sentence, because both clauses are independent. (They can act as sentences on their own.)
Option D: The given sentence expresses two contrasting ideas, "train hard" and "you will not make it", they're alternative because
training hard gets you your desirable result, and "you will not make it
up" isn't the desirable result. Hence, an alternative conjunction is required.
'If' expresses a conditional, hence it shall be replaced and option D is incorrect.
Option C: 'Else' means 'to face a consequence', it's an alternative conjunction,
which is the appropriate option here, hence option C is correct.
Option A: 'Unless' is a subordinating conjunction used for condition-related
sentences, similar to 'if', and the sentence isn't conditional, hence
option A is incorrect.
Option B: 'Though' is used in complex
sentences whereas the given sentence is compound. Hence option B is incorrect.