Question

Biologists are studying tropical plants to determine how far north each type of plant can survive. What is the population?

Answer 1

It is tropical plants

Answer 2

It is the tropical plants. The text asks about population which is defined as the total number of an object or an entity. In scientific researches, samples are drawn from population to explore either qualitatively or quantitatively a phenomenon. Moreover, biologists study plants as subjects in this study.