The movement of earth's tectonic plates relate to many factors, including density differences in the plates and
a. convection cells in earth's atmosphere.
b. convection currents in earth's mantle.
c. the number of species living on each plate.
d. how closely humans monitor plate movements.

The answer is including density differences in the plates and B. convection currents in Earth's mantle.

Let's look into the options one by one:

A. convection cells in Earth's atmosphere.

-This is false.

- By no means does the Earth’s atmosphere affect the movement of plates, given that they do not even touch the place let alone applying force or changing it.

B. convection currents in Earth's mantle.

-This is true.

- Under the convection currents in Earth’s matle, the plate will be moved accordingly. For example, under the rising convection current of magma, the plates will be pulled apart, forming a divergent plate boundary.

C. the number of species living on each plate.

-This is false.

- The number of species living on it plate does not affectthe plate movement by any means.

D. how closely humans monitor plate movements.

-This is false.

- Although the plate movement can be monitored to predict earthquakes and volcanic eruptions, there are currently no technology to control the plate movemnet, given a plate could extend to several continents.

Therefore the answer is B. convection currents in Earth's mantle.

Hope it helps!

windward