Brazil (1) plays rank 4 in semi-finals (Round 4 - magic number is 5)

But, we know all even numbered matches of round 1 and all even ranked teams lose in round 1, hence lets trace rank 4's path to know which is the LOWEST ranked team that can replace it.

Round 1: 4 + 29 ("Magic number for round 1 is 33=4+29”) $=>$ 29 qualifies and takes 4's place

Round 2: 4 + 13 ("Magic number for round 2 is 17=4+13”)

But, now the round 2 match is not 4 v/s 13 but 29 v/s 13 $=>$ 13 is a better rank team and no upsets occur in round 2 so 13 qualifies and takes 4's place in the next round.

Round 3: 4 + 5 ("Magic number for round 3 is 9=4+15”)

But now here 4 is replaced by 13 so its 13 v/s 5. The winner of this match meets 1 in the next round(Semi-finals). We have the liberty to choose any of these two teams but the choice has to be of the LOWER team hence, choose rank 13 =Switzerland as the ANSWER (a)