Final answer:
Higher heritability would increase the rate of evolution by natural selection for a particular trait. On the other hand, lower mutation rate, smaller population size, and less variation in fitness would not directly increase the rate of evolution.
Step-by-step explanation:
The variable that would increase the rate of evolution by natural selection for a particular trait is higher heritability. Heritability refers to the proportion of variation in a trait that is due to genetic factors, and traits with higher heritability are more likely to be passed on to future generations.
In contrast, lower mutation rate, smaller population size, and less variation in fitness would not directly increase the rate of evolution by natural selection for a particular trait.