a,
If you go to the Miss A concert you give up a benefit of $190 (willingness to pay to see EXO). But you save the $180 for the ticket. Therefore, the opportunity cost is 190 – 180 = 10. There is no such thing as “free lunch”.
b,
If you also got the EXO ticket for free, then your opportunity cost of seeing Miss A would be $190 which is higher than before.
c,
The 3rd alternative will NOT affect your answers. Only the second best option is taken into account when you calculate the opportunity cost.