A – Wrong
The cause of psychotic illness could have been something else rather than the increase in cannabis use. (age, memory etc.)
B – Wrong
This provides other factors that could cause psychosis which is the opposite of what we need to answer the question.
C – Correct
This clearly suggests that increase in strength of cannabis causes an increase in psychosis among users.
D – Wrong
Age is irrelevant
E – Wrong
This does not strengthen the link made in the text.