Which type of stimulus is an allergen, such as pollen?

A. response to a stimulus

B. external stimulus

C. internal stimulus

D. inborn stimulus

I think the correct answer from the choices listed above is option B. External stimulus is the type of stimulus an allergen, such as pollen, is classified. This is because this allergen is obtained outside the body. So it must be external. Hope this answers the question. Have a nice day.