Step 1: Understand the underlined phrase.
The underlined portion is "who is the best in the country by a mile". The idiom "by a mile" normally means "by a large margin" and is used to compare one thing against others, for example, "he won the race by a mile".
Step 2: Spot the logical problem.
"The best" is already a superlative; it represents the highest possible position, so there is nothing left to measure a margin against. Saying someone is "the best... by a mile" is redundant and illogical, because a superlative does not admit degrees the way a comparative does.
Step 3: Test the remaining options.
Replacing "by a mile" with "for a mile", "with a mile" or "within a mile" does not fix the underlying redundancy; each of these merely swaps one illogical distance phrase for another equally illogical one attached to a superlative.
Step 4: Choose the cleanest correction.
Simply dropping the redundant phrase gives a grammatically sound and logically clear sentence: "When you leave out a player who is the best in the country, the people need to know the real reasons..."
Step 5: Conclusion.
Option (b), "Who is the best in the country", is the correct replacement.