Which ones are long day plants?

Short day plants are those plants that begin flowering when the days are shorter than their critical day length. They require long period of darkness and short period of light to flower. Long day plants are those plants that begin flowering when the days are longer than their critical day length. They will flower when they are exposed to short period of darkness and long period of light. Long day plants flower in summer and short day plants flower in spring. Examples of short day plants are rice, tobacco, cotton, soyabean, Xanthium. Examples of long day plants are lettuce, beet, oat, wheat, spinach, poppy. 

Thus, the correct answer is option B.