The statement that best explains why a hot air balloon rises is A. As the air inside the balloon is heated its volume increases, inflating the balloon more fully. Having a greater volume makes it less dense than the surrounding air so it floats upward.
Interestingly, Jacques Charles for whom Charles' law was named was a balloonist. He launched the first unmanned hydrogen balloon in the late 1700s. The hydrogen balloon rose in the atmosphere for the same reason, it was less dense than the surrounding air but due to hydrogen's lighter mass rather than its temperature.
Charles, a French chemist, experimented with gas volume and temperature by filling balloons with different gases to the same volume then lowering the temperature of all of the balloons by the same amount. He found that the volumes of all of the gases decreased by the same amount.