These observations might allow us to infer what difference between the conditions of these ecosystems ?

Your answer would be A. The amount of available sunlight. A plant's leaves absorb the sunlight for the plant to use, so the different ecosystems would cause the leaves on the different plants to be different.
Ex: Cacti in the desert have plenty of sunlight all day so they have very small leaves (spines), whereas a plant in the jungle that has to compete with many other plants for sunlight, for instance the cardoon, has big leaves.

Ten percent of energy is transferred from sun to the level indicated.