The correct answer is C. Relation between organisms and their environment.
Ecology is the scientific study of the relations that living organisms have with one another and with their physical environment. It is a broad subject that encompasses the study of ecosystems, communities, populations, and individuals.
Option A is incorrect because cell formation is the study of how cells are created and how they function. This is a topic in biology, not ecology.
Option B is incorrect because birds are a type of organism. Ecology is the study of the relationships between organisms and their environment, not the study of individual organisms.
Option D is incorrect because tissues are groups of cells that work together to perform a specific function. This is a topic in biology, not ecology.