The critical detection point in a Physical Protection System (PPS) occurs when the remaining distance or time an adversary must cover before reaching the target exceeds the security response time. This ensures that detection happens early enough for responders to act and intervene before the attacker reaches the asset.

B and C refer to detection and interruption probabilities, which are important metrics but not the defining threshold for the critical detection point.

D (Remaining delay time) factors into the protection layers but does not define the detection threshold.