C should answer "No".


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


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.