This provides some support for the criminologist's statement. If it is true that many people rob because they enjoy the risk, then imposing harsher prison terms would, by increasing the risk, potentially increase the thrill, rather than deterring people from robbery.

This might seem weak, but in the absence of additional information, this gives some support to the criminologist's claim. We're told that rates of another crime did not fall as a result of longer prison time, which is potentially analogous to the case of robbery. Without a reason to reject the analogy, this makes the criminologist's claim slightly more likely to be true.

This is irrelevant. Without knowing how the decreasing sentences have affected robbery rates, this information doesn't help us either strengthen or weaken the criminologist's claim that increasing prison time will not have a significant effect on robbery rates.

This strengthens the criminologist's claim. Increasing the length of the potential prison sentence won't deter robbers, since most people who commit robbery are convinced they won't be caught anyway.

This strengthens the criminologist's claim. If robbers aren’t aware of what the likely sentence for robbery is anyway, then it makes sense that increasing prison time won't deter them, because they won't be aware of the change.