Describe the experience of Entrainment you had recently as a result of the time change to Daylight Saving Time.

The answer would be b.
A population is the number of organisms of the same group or species. 
a describes ALL the organisms in the forest, but a population is the amount IN A SAME GROUP OR SPECIES, but a is talking about ALL THE SPECIES, so NO NOT A.
C describes BOTH plants and animals but a population is only one group. 
D describes a nonliving part of an organism's environment, but an organism only consists of the living environment.
B describes a population. The amount of squirrels in an area is a population.
REMEMBER, A POPULATION CONTAInS ONLY ONE GROUP OF THE TYPE OF SPECIES.