The factors that shift the MC curve cause the market supply curve to shift. In the short run, basically two factors can cause the market supply shifts to the rightward:
(i) A decrease in production costs shifts the market supply curve rightward. For example, a technological innovation which leads to a rise in labor productivity decreases the unit labor cost and causes the market supply curve to shift to the right.
(ii) In the long run, an increase in the number of firms, i.e., new firms entering the industry, shifts the market supply curve rightward.

The correct answer is B.