The NHL rule book states the following regarding eligible players for game day
5.1 Eligible Players - A team shall be composed of 20 players (18 skaters and two goalkeepers) who shall be under contract to the Club they represent. For the purposes of these playing rules, any reference to "player" shall refer to both skaters and goalkeepers. Any reference to “goalkeeper” shall mean that the section of the rule is specific to goalkeepers.

Although you have "A" = 21 as the correct answer, 20 is actually the correct response ... cheers though good quiz