SOLUTION :
Total marks of 10 failed students $=28 \times 10 =280$
Total marks of 50 students $=2800$
$\therefore$ Total of 40 passed students $=2800-280 =2520$
$\therefore$ Average marks of 40 passed students $=\frac{2520}{40}=63$
So, The correct option of this question will be (C).