How do all organisms begin? as one single cell
It depends on whether the adult organism is single-celled or multi-celled.
as a collection of cells
as a group of daughter cells

1. / A.

Explanation:

Its the first answer (as one single cell)

Because that's how all organisms begin life as a single cell some organisms begin life as a single cell and others as multi-cellular.

the correct answer is:

a population changes in response to a change in the environment.