Before I address this question, I want to point out that this is a poorly written passage. It is riddled with grammatical and factual errors:
- the Seneca Falls Convention happened in 1848, not 1948
- "considered to be" is not idiomatically correct
- "100 of the approximate men and women" makes no grammatical or logical sense
- "Alice Paul was forced to resign... and started her own..." needs a comma before "and," because these are separate independent clauses

It is a bad idea to study from any source that takes so little care in writing its passages. You should only study from reputable sources that are very like the GMAT.

Now to your question... This question asks us to find the Primary Purpose. When looking at RC questions, it's a good idea to look for 4 wrong answers, rather than look for the right answer. Think about what you can prove wrong.

That said, it's a good idea to have your own idea of the primary purpose before you begin. Here, it might be: Showing the various events that led to a political outcome: women's right to vote.

A. Political figures are mentioned, but the primary purpose isn't to detail their actions. It's to show the results.
B. Again, organizations are mentioned, but it's not about them. It's about what they caused to happen.
C. This passage mentions that people had different viewpoints, but doesn't go into depth about what they thought and why. It's centered on actions and outcomes. It's also not clear that everyone had a common goal - some people thought women deserved the right to vote; others did not.
D. Yes, this passage details all of the relevant events that led to the outcome of women's suffrage. CORRECT
E. It's not just about that one organization; it's about the whole movement.