Answer: B). External stimulus
A stimulus can be defined as any substance or particle that initiates quick action or response in the organism. An external stimulus can be defined as stimulus, which is initiated by foreign agent belonging to the external environment in which the organism interact. Pollen is a fine powder, produced by the flowers. It is a male gamete in flowers. It fertilizes the female gametes in flowers that is ovary, to form seeds and fruits. The pollen exhibit in the form of small grains. Humans have experienced allergic response due to pollen. Pollen being an foreigner agent for the human body, when inhaled, activates the immune system, to fight against it, therefore, it is an external stimulus. This results in an allergic reaction, to expel out the pollen particles from the body. This includes symptoms like sneezing, stuffy nose and watery eyes.