Answer: D) harmless plant proteins
Pollen is a male gamete, which is produced by the flowers in order to conduct fertilization of female gamete or ovule. The pollen contains proteins which acts as an antigen. When these proteins or antigens are inhaled by the living organisms like humans it creates an allergic response in them. This occurs due to the fact that, when body receives a foreign antigen, the body immune system gets activated and it produces antibodies against it, to expel it out of the body by sneezing.
According to the above information, the culprit is affected by harmless plant proteins, which will originate an allergic response, causing Juan to sneeze when he sniffs the morning glory flower.