Question

an ecological community in which populations of plants or animals remain stable and exist in balance with each other and their environment at an optimal level
primary succession
climax community
opportunistic species
pioneer species

Explanation:

The description is about an ecological community in stable balance, and the term "Climax Community" matches this definition (a stable ecological community at the end of succession). Other terms: Primary Succession is the process of community development on new sites, Pioneer Species are the first species in succession, and Opportunistic Species are adapted to disturbed environments, not stable ones.

Answer:

Climax Community