Question

what type of stimulus is an allergen such as pollen?
external stimulus
internal stimulus
response to a stimulus
inborn stimulus

Explanation:

An external stimulus originates from outside an organism's body. Pollen is a substance that comes from the environment, outside the human/animal body, and triggers an allergic response. Internal stimuli come from within the body, a response is the reaction to a stimulus, and "inborn stimulus" is not a standard category for this context.

Answer:

external stimulus