I'm not certain if I'm right but I think it's C.
Equilibrium is a chemistry term that means "at the same rate" so I assume it's the word that's out of place here.
Answer:
our planet
Explanation:
Forests. Forests are large areas covered with plants.
Grasslands. Grasslands are regions dominated by grasses.
Deserts. Deserts are areas that receive very little rainfall.
Mountains and Polar Regions. Earth’s polar regions (the Arctic in the north and the Antarctic in the south) and tall mountains are extremely cold.
Aquatic Habitats are seas, oceans, rivers, etc.