The true statement is:
A) An ecosystem is classified by factors such as temperature and precipitation.
This statement is accurate as ecosystems are influenced by abiotic factors like temperature and precipitation, which help define the types of habitats and the organisms that can thrive in them.
While option C is also somewhat true, it is less precise than A. Biomes do contain ecosystems, but it's important to note that biomes are defined by broader climatic and geographical factors, whereas ecosystems are more localized communities of living organisms interacting with their environment.
Option B is misleading and incorrect, as almost all biomes are significantly larger than what could be observed under a microscope.
Option D contains a typographical error and is also a bit simplistic in its definition of an ecosystem.