The statement that is true of mutation is:
It is a random process that leads to variation.
Mutations are changes in the DNA sequence of an organism's genome, and they occur randomly. These mutations can introduce new genetic variations within a population, which can be crucial for evolution and adaptation. The other statements do not correctly describe mutation: random assortment and crossover events are mechanisms of genetic recombination during meiosis, not mutations, and mutations do not make a population immune to environmental change. They may contribute to variation that can be advantageous or disadvantageous depending on the environmental context.