+1 for A

B - Incorrect. The argument doesn't mention about a player becoming successful
C - Incorrect. Same as B. Argument doesn't mention about successful players
D - Incorrect. This is too strong. It can't be a must be true statement
E - Incorrect. There can be a dilemma between A and E. E is the trap answer. The statement tell us that all unsuccessful players have ignored coaches advice. But it's not a must be true statement because a player can be unsuccessful for many reasons in the world

A - Correct. This is the best available option as it says that not all but maybe only a few players might have ignored coaches advice.

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A. Not all professional football players who played the sport as youths ignored the advice of coaches.
Will Keep it for now !!

B. The more attentive a professional football player is to his coaches' advice, the more likely he is to be successful.
Additional Information/Out of scope
In a Must be true type , we cannot introduce any other information

C. The most successful professional football players played the sport as youths.
There is a difference between played the sport as youths and began playing football as youths
out

D. No professional football player who learned the sport later in life will ignore his coaches' advice.
I would count it as additional information
so out-

E. All unsuccessful professional football players have ignored the advice of coaches.
The set of all unsuccessful professional football players is large, we cannot say all were unsuccessful because they ignored the advice.
out-

A