Answer:
The correct answer is : temporal isolation.
Explanation:
Temporal isolation is a one type of prezygotic reproductive isolation in which the the prevention of mating of the two species takes place due to the time difference of breeding, this could be day difference, season difference, or years difference.
Here in this example the reproductive isolation is taking place due to the difference in their mating time, where L. graminifolia flowers in the early spring and L. canadensis in different season of summer.
Thus, the correct answer is : temporal isolation.