Final answer:
Heritability and variation in fitness are key factors that increase the rate of evolution by natural selection for a specific trait, as they ensure transmission of advantageous traits and promote differential reproductive success.
Step-by-step explanation:
The rate of evolution by natural selection for a particular trait would most likely be increased by heritability and variation in fitness. Heritability is essential because it ensures that the traits which confer an advantage are passed down to the next generation. Meanwhile, variation in fitness among individuals within a population promotes differential survival and reproductive success, which is the essence of natural selection.
A lower mutation rate does not increase the rate of evolution, because mutations are a critical source of genetic variation necessary for evolution to occur. Likewise, a low variation in population size typically does not contribute to an increase in evolution rate; in fact, a smaller gene pool can limit the variation upon which natural selection can act.
Therefore, heritability ensures the advantageous traits are inherited, and variation in fitness ensures that those traits can lead to more offspring (and thus a greater representation in future generations).