Kelsey's post here is relevant: viewtopic.php?p=82968#p82968
In short, 5 people must be selected, via the scenario.
For question #2 specifically, if exactly one chemist is selected, as in the local condition for the question, we should consider what other categories of scientists are selected. If more than one botanist is selected, we'd get only one zoologist, via the second rule, so it's either 1 botanist (and thus 3 zoologists), or all 3 botanists (and 1 zoologist). The problem is, M is selected, so P and R are selected (last rule), so the 1 zoologist situation doesn't work. So 3 zoologists must be selected, which is all of them. That leads to answer choice (E).