Concept:A population is a group of organisms of the same species living in the same area at the same time.
Explanation:In ecology, a population refers only to individuals of one species in a defined habitat.
Option A, Tilapia in a lake, describes a single species, Tilapia, in one specific location, the lake.
Options B, C, and D each involve two different species, so they represent communities, not populations.
Therefore, the phrase that correctly describes a population is the number of Tilapia in a lake.
Answer:A. Tilapia in a lake