Solutions
Given that only one among A, B and C is truthful.
Case 1: A is truthful
A: turn left → it means we need to turn left
B: do not turn left → it means we need to turn left
C: go straight → it means we need to turn left or right
Conclusion: A is truthful, and we need to turn left
Case 2: B is truthful
A: turn left → it means we need to turn right or go straight
B: do not turn left → it means we need to turn right or go straight
C: go straight → it means we need to turn left or right
Conclusion: B is truthful, and we need to turn right
Case 3: C is truthful
A: turn left → it means we need to turn right or go straight
B: do not turn left → it means we need to turn left
C: go straight → it means we need to go straight
Conclusion: The statements are contradicting each other and hence the C is not truthful.
Final conclusion: As it satisfies first two cases, he will not be able to decide between going left or right.