What does change over time and space?
A. Ecological system
B. Ecosystem
C. Ecological function
D. Environment

Hint: Ecosystem is the basic structural and functional unit of nature. In an ecosystem, a living organism interacts among themselves and with other physical factors. All components in an
ecosystem have important roles in maintaining a balance of an ecosystem and to maintain its
ecological functions.

Complete answer: An ecosystem consists of both living and nonliving things. Nonliving components of the ecosystem are called abiotic components, and the living components are called biotic components. Biotic components include plants, animals, and other living organisms. Meanwhile, abiotic components include non living things like soil, water, and air. Each and every organism in an ecosystem lives in constant interaction with both biotic components and abiotic components, and every organism has a role to play in the smooth and efficient functioning of an ecosystem. But sometimes due to certain factors, this balance is disrupted and the ecosystem is destroyed. Ecosystems are susceptible to undergo changes over time and space. It occurs in parallel with the changes in species composition in that ecosystem, which is called ecological succession. Some species are added to an ecosystem while some others disappear along with the changes in their surroundings. It includes both changes in vegetation along changes in species diversity of organisms. Know that the sequence of communities that changes successively in a given area is called seres.
Hence the correct option is B.

Note: the necessity of protecting ecosystems as it has a great impact on our lives. Ecosystem services to mankind include pure air and water, fertile soil, pollination of crops, maintenance of biodiversity, and lots more.