To find out how much the boy will have left after paying his mother, we need to subtract the amount he owes from the amount he earned.
He earned: $12.50
He owes: $5.75
Now subtract the amount he owes from what he earned:
\[ 12.50 - 5.75 = 6.75 \]
So, after he pays his mother, he will have $6.75 left. Since $6.75 is not among the options given, the answer is none.