This would weaken the argument. It provides an alternative explanation for the psychologists' observations. Drivers aren’t necessarily leaving more slowly because they feel more possessive, but because they feel pressure from the waiting cars, and even more pressure when those cars honk at them.

This doesn't weaken the argument. If anything, this would slightly strengthen the argument by undermining the alternative possibility that drivers are slower in general when they know others are watching them.

Notice that, while this explains why drivers took longer to leave the space when cars were waiting versus when no cars were waiting, it doesn’t explain why honking made the drivers take even longer. In other words, this answer choice doesn't provide an alternative explanation for that aspect of the phenomenon, and so the author's hypothesis about possessiveness explaining these observations is untouched. We need an alternative explanation that accounts for the whole phenomenon.

This answer choice suggests that in shopping mall parking lots, cars are more--or less--likely to be waiting for spaces than in other parking lots. But we're not interested in how often cars tend to wait for parking spots, just in what happens when they do, and why. This answer choice doesn't weaken the argument.

This addresses why drivers took longer to leave when a waiting driver was honking. It doesn't explain why drivers took longer to leave when the waiting drivers were waiting quietly. This doesn't account for the whole phenomenon, so doesn't successfully weaken the author's hypothesis that possessiveness is the root cause of this phenomenon.