The correct option is C.
Since a person is standing in a room that has 3 locked doors and there are three colored keys on a keyring: one red, one yellow and one green, and they have been provided the following information:
- Each of the green and the yellow keys can unlock only one door, but the red key can be used to unlock two of the doors
- Once opened, the middle door is followed by an additional door whose key is not among the three keys
- The first door can't be unlocked by the red key
- The last door can't be unlocked by the green key
And the person has only one attempt to use the correct key and exit the room, to determine which would be the correct attempt, the following logical reasoning must be performed:
The first door cannot be unlocked with the red key, since it opens two doors, it can open the second door and the third door.
The last door can also be unlocked with the yellow key, then: first door opens with the green key, second with the red key and third with the red and yellow keys.
Since a key is missing to open the middle door, two options remain: open the first door with the green key or the last door with the red or yellow keys.
Therefore, the correct option is C: Use the green key to open the first door.
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