Final answer:
The important change in warfare during the revolutionary era was the use of mass armies of citizen-soldiers. Option B is correct.
Step-by-step explanation:
The important change in warfare during the revolutionary era was the use of mass armies of citizen-soldiers. Before the revolutionary era, cavalry were the most effective troops on the battlefield due to their speed and ability to deliver fatal blows or trample infantry. However, with the invention of gunpowder and guns, cavalry lost their advantages as they could be killed before reaching opponents on the ground.