The correct answer is C) L. The knot illustrated as L is the constrictor knot, which is a secure knot but is only fully tightened and locked when there is tension or strain on the line. Without tension, the constrictor knot can work itself loose. The other knots shown (H, I, and P) are either not as secure or do not require tension to maintain their shape and grip. Therefore, the constrictor knot (L) is the only one in the illustration that is secure only when there is a strain on the line.