Final answer:
Voting rights were extended to women in 1920 with the 19th Amendment, while the voting age was lowered to 18 in 1971 by the 26th Amendment. The difference in time frames was largely due to social movements; Women's suffrage was a long campaign, whereas the change for youths was influenced by the context of the Vietnam War.
Step-by-step explanation:
Comparing the expansion of voting rights to women with that of 18-year-olds, we see that the franchise was extended to women nationally in 1920 with the ratification of the 19th Amendment, after decades of activism and gradual changes at the state level. For 18-year-olds, the voting age was lowered from 21 to 18 much later, in 1971, with the adoption of the 26th Amendment. This change was largely influenced by the cultural context of the Vietnam War, wherein young individuals were eligible for the draft at age 18, creating momentum for granting them the right to vote.
The most likely factor contributing to the different time scales of these expansions is c. Social movements. The women's suffrage movement, beginning in 1848 and culminating with the 19th Amendment in 1920, was driven by persistent advocacy, while the change for the youth vote in 1971 was partly due to the social and political climate surrounding the Vietnam War, and the felt incongruity of young men being drafted without having a say in the electoral process.