The context says Nora alone carries the key. Which sentence makes the only one the antecedent of the relative clause?
Nora is the only one of the guards who carry the vault key during every shift.
Nora are the only one of the guards who carries the vault key.
Nora is the only one of the guards who carries the vault key today.✓Правильный ответ
Nora is the only one of the guards who carrying the vault key downstairs.
Объяснение
Nora is the only one of the guards who carries the vault key uses singular carries because the relative clause identifies Nora alone. Carry can be standard when who instead modifies the embedded plural guards, but the premise rules out that attachment; are and carrying cannot supply the required finite agreement pattern.