Puzzles/Logic puzzles/Knights, Knaves & Spies II/Solution

Puzzles | Logic puzzles | Knights, Knaves & Spies II | Solution

C should answer "No".

Reasoning

B is either a knave or a spy. If B is a spy, then A is truthful and is therefore the knight.

 A - Knight
 B - Spy
 C - Knave

On the other hand, if B is the knave, there are two possibilities:

 A - Spy
 B - Knave
 C - Knight

or

 A - Knight
 B - Knave
 C - Spy

If C is either the knave or the knight, his answer to the question will be "No", and so the judge will not be able to draw a conclusion. On the other hand, C can answer "Yes" only if he is the spy.