There are two species of larkspur that have two different size flowers; Delphinium decorum has large, fully opened flowers and Delphinium nudicaule has small bud like flowers. These two species are both fertilized by insects, but they cannot be fertilized by the same type of insect because the smaller flower of D. nudicaule will not permit larger insects to reach the nectar of the flower and the larger flower of the D. decorum is typically fed on by larger insects.
Which type of isolating mechanism would this be?

a) Postzygotic Isolation
b) Temporal Isolation
c) Behavioral Isolation
d) Habitat Isolation
e) Mechanical Isolation