There is no reason (that life may exist) P (in great profusion) Q (why we should not be willing to think) R (in other worlds) S The correct sequence should be

Since, the sentence starts talking about the reason; R will be the continuing statement as it goes with the flow of the statement. The continuing statement talks about the existing of life i.e. P. Thus, all other options are eliminated. The correct formation would be, ‘There is no reason why we should not be willing to think that life may exist in great profusion in other worlds.’