The correct answer is C. vociferous.
Vociferous means “characterized by loud and insistent demand or protest.” In the context of the question, the opposition is making a loud and insistent demand for the bill to be put to a vote. This suggests that they are passionate about their position and are not afraid to speak up.
The other options are not as accurate. Courageous means “having or showing the quality of being willing to do or face dangerous or unpleasant things.” Brave means “having or showing great courage.” Powerful means “having or showing great power or strength.” While the opposition may be courageous or brave in their demand, the word that best describes their actions is vociferous.