Which of the following has the smallest pollen size?

The choice identifying Urtica (nettle) as having the smallest pollen size is supported by the characteristics of these plants.

Nettle is known for its relatively small pollen grains, which typically range in size around 10-20 micrometers. Smaller pollen grains like those from Urtica can be easily transported by wind, which is often the primary mode of dispersal for plants that are adapted for wind pollination.

In comparison, the other options have larger pollen grains. For instance, Pinus (pine) produces larger pollen grains, often exceeding 20 micrometers, and can range up to 50 micrometers. Quercus (oak) also has relatively larger pollen grains, often in the range of 20-30 micrometers, while Platanus (plane) can produce pollen that can be substantial in size, generally also larger than that of Urtica.

Thus, recognizing the characteristics of pollen grains in these plants, Urtica (nettle) stands out for its smaller pollen size compared to the others listed.