The paragraph is one long reported conditional sentence. Its skeleton is: introductory phrase (i), reporting clause, if clause, result clause. Sentence i and sentence vi are locked, so the four middle sentences must fill that skeleton. Once you notice that ii ends in "that", v begins with "if", and iv is the only clause with a subject and a future verb ("they will live"), the arrangement is forced.
The completed paragraph reads: "On the basis of experiments with rats, health experts here say that if humans eat less, exercise more and consume vitamins, they will live up to 100 years or more and be vigorous in their eighties and nineties."
Let's summarize:
The middle sentences therefore run ii, v, iii, iv, and option (B) is correct.