If we paraphrase the passage, it might look like this:
- people think that maternity leave policies were driven solely by market forces, because...
- the law about it was was passed in 1993
- but many employers enacted policies in the 70s and 80s, long before the law
- however... there was a version of the law put forward in 1972
- it was widely publicized, so perhaps employers started getting on board because they heard about it
- the law was struck down in 1973, but employers kept the policies
- because the law was struck down, people forget that it might have had an effect
- it might not be just the market that had the effect

Or, boiled down even further...
- people think that X caused Y
- however, there was a Z that could have caused Y, but people forgot about it

For the main idea, we want something that captures the idea of "people tend to think it's one explanation, but it might be this other thing they forgot about."

The primary purpose is to

A. present an alternative to a commonly accepted explanation for a phenomenon

Exactly. We know that "many politicians, [etc]... emphasize the role of the labor market." So that's the commonly accepted viewpoint. The alternative is that the 1972 law might have actually impacted company policies, even though "perhaps... politicians and scholars failed to recognize its effects."


B. reexamine a previously discredited explanation for a phenomenon in light of new evidence

There is no discredited explanation. There is just the commonly accepted one, and the alternative one that the author puts forward. There is also no new evidence, just overlooked evidence.


C. criticize politicians and scholars for failing to anticipate a phenomenon

Nope. "Criticize" is WAY too strong / opinionated. Also, "anticipate" would be future-looking, and this passage stated that politicians and scholars failed to look backwards and realize the effects of something.


D. correct a common misconception about the impact a phenomenon has had on a government policy

The author is not definitively stating that people are misinformed, only that "PERHAPS" they've overlooked something. Also, the causation is backwards. The government policy affected the phenomenon (of maternity leave), not the other way around.


E. analyze the ways in which a phenomenon has changed over time in response to market forces

It's not about the market forces. The purpose of the passage was to show that it was the government's law, not the market forces, that shaped this phenomenon.


For main idea questions, trying paraphrasing the passage, and abstracting the structure to the highest level. Don't get lost in the details - the specific nouns. Find the moments of transition and contrast.

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