Answer:
C. a pivotal
Step-by-step explanation:
The word "pivotal" refers to something that is quite important, and that has consequences that are significant and long-lasting. In this case, we learn that 1775 was an important year because it was the year in which the colonists' grievances finally gave way to armed conflict. Therefore, we could consider this a "pivotal" year due to the long-lasting consequences of it.