A pool factor typically refers to conditions that attract people to a particular place, often in the context of migration. Among the options you've provided, "A better job" (option A) is the best example of a pull factor, as it represents an economic opportunity that draws individuals to a different location.
The other options—B "horse," C "climate," and D "war"—are either ambiguous or represent other concepts (like push factors, in the case of war and potentially unfavorable climate).