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.