Final answer:
Desert ecosystem is the type of ecosystem that is expected in an area where evaporation exceeds precipitation and mean annual rainfall is below 100 mm. Therefore, the correct option is A.
Step-by-step explanation:
In an area where evaporation exceeds precipitation, and mean annual rainfall is below 100 mm, the expected type of ecosystem is Desert ecosystem. Deserts are characterized by arid conditions with low and sporadic rainfall. The imbalance between evaporation and precipitation results in dry and often harsh environments, leading to the development of desert ecosystems.
These ecosystems showcase unique adaptations in flora and fauna to thrive in water-scarce conditions. Unlike tropical rainforests (b) that experience high rainfall, tundra ecosystems (c) known for cold climates, and savanna ecosystems (d) with a seasonal distribution of precipitation, deserts are specifically defined by their arid nature, making option a the most appropriate choice for the described conditions.
Therefore, the correct option is A.