there are mechanisms that prevent pollination of one plant species by pollen from another. this would prevent
a) the formation of hybrids between the pollen donor and the pollen receptor
b) a waste of pollen
c) pollen from attempting to develop on the stigma of an incorrect species
d) bees from pollinating indiscriminately
e) all of the above would be prevented
e is probably the best answer for an introductory course. However, a is the best answer overall.
consider d) Bees are not the only vector for pollen, trees are usually pollinated by wind, as are grasses
b) is silly, pollen is in great abundance. Have you ever parked under a pine tree?
d) Bees go from flower to flower, for nectar or pollen. I am not certain fidelity to flowers is evolutionary important to plants, as most are not pollinated by bees.