The question does not ask us to find the MAIN POINT of the argument, rather it asks us to find out how the main point of the passage is MADE or SUPPORTED.
Main point of the passage is:
Most discussions of the factors contributing to improvements in public health greatly underestimate the influence of the values held by individuals.
Support for the main point is:
1) This influence is indicated by the fact that the astonishing decline in mortality from infectious disease during the past century was primarily due to an improvement in living conditions.
2)To a substantial degree, these improvements depended on the emphasis by an increasing share of the population on cleanliness, prudence, and moderation.
Option B is the correct answer because
emphasis by an increasing share of the population on cleanliness, prudence, and moderation -> improvement in living conditions -> decline in mortality from infectious disease during the past century -> influence of the values held by individuals
Quote:
(B) presenting a set of related cause-and-effect assertions