Most discussions of the factors contributing to improvements in public

I don't think that is the reason because option B also mentions set of cause and effect assertions, but only one cause and effect relationship is provided in the passage.

I thought the same way too.

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yes... we cannot find different examples in argument.
hence D is obviously out

The effect is firstly mentioned, followed by the cause for it. The cause is then analyzed for it being caused. Hence, set of cause and effect assertions. I had boiled down to B/D too.

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