Final answer:
The most significant characteristic of a population is its number, based on the population density and the area the population occupies. Density plays a crucial role in population ecology, affecting resource competition and reproductive success. The correct option is a.
Step-by-step explanation:
The most significant characteristic of a population is its number, which is in turn a function of the density (individuals/unit area) and the area occupied. Population density is defined as the average number of individuals per unit of area or volume.
Therefore, the size of a population depends on both its density and the area over which it is spread. For example, two populations with the same number of individuals might have different densities if one occupies a larger area than the other.
Understanding the density of a population is crucial, as denser populations can indicate different ecological and biological patterns, such as the potential for competition for resources or the ease of finding mates. The correct option is a.