First, let's deconstruct that first sentence a little bit.

ALTHOUGH the unions that emerged (detail detail detail) embraced nondiscrimination and inclusion (or good things), the role of women reflected the "gender ideology" of the period.

What this really means:

ALTHOUGH X was true in theory, something different was actually going on in practice. The unions officially embraced certain policies, but the actual day-to-day roles of women in particular were a bit "stuck in the past" and, presumably, the women were still not completely included and still experienced discrimination despite the lofty policies of the unions.

The right answer is worded a bit confusingly, but the wrong answers are all definitely wrong.

A) says that the "gender ideology" prevented women from making contributions to the establishment of unions, but we don't know anything about this (and, in particular, we're not talking about who started, or established, the unions)

B) says that the "gender ideology" resulted from the marginalization of women - but that's backwards. If anything, the marginalization of women was a result of the gender ideology.

D) says that gender ideology's primary tenets were nondiscrimination and inclusion, but the passage says that the unions embraced those two things. It doesn't connect the concept of gender ideology with those things.

E) says that gender ideology's effects were mitigated by the growth of the unions. Although the end of the first paragraph talks about the "peak" of the unions - that's not where we're talking about gender ideology. That part of the passage has moved on to talking about the community base and women's contributions.

C) says that the gender ideology had a significant effect on the advancement of women's issues in the unions. If the unions did embrace non-discrimination and inclusion generally, BUT women still weren't really being included, and that non-inclusion "reflected the gender ideology of the period" - well, then that gender ideology did have an impact on how women were perceived and how women's issues were handled within unions. The "prevailing gender ideology" is why the women were still having problems despite the fact that the unions were supposedly embracing non-discrimination and inclusion.