Final answer:
The correct word to complete the sentence is 'or', which indicates two alternatives - winning or losing the race. It reflects the idea that personal effort is more important than the outcome.
Step-by-step explanation:
The correct conjunction to use in the sentence 'Whether you win this race or lose it doesn't matter as long as you do your best' is 'or'. The use of 'or' in this context indicates the presence of two alternatives or possibilities - winning or losing the race. It emphasizes the importance of effort over the outcome. The question highlights the concept that the journey and the exertion put forth are equally, if not more, important than the actual result, aligning with the philosophy that personal excellence is defined by one's dedication and hard work rather than just victory.