This answer choice tells us that there was a clear increase in the number of Californian students who dreamed about earthquakes after they experienced one. This strengthens the conclusion that the earthquake caused these dreams.

This would weaken the author's argument. Perhaps the students in Ontario did dream about earthquakes, but didn't remember those dreams because they hadn’t studied dream recollection.

This just tells us that perhaps the Californian students had prior experience of earthquakes. This neither strengthens nor weakens the author's claim that experiencing an earthquake can cause dreams about earthquakes.

Irrelevant. We're interested in how many participants dreamed about earthquakes at all, not in how many dreams each participant had.

This suggests that some of the Ontario students may have heard about the earthquake and dreamed about it afterward. But this doesn't change the fact that none of the Ontario students had directly experienced an earthquake, and that the majority of them didn't dream about earthquakes. This doesn't strengthen the author's conclusion.