Natural selection acts by taking advantage of natural variations in the traits of organisms within a population what is the ultimate source of this variation

The answer is the mutations within the genetic code. 

Natural selection is one of the basic mechanisms of evolution by which species adapt to their environment in order to increase their chance of survival and reproduction. In natural selection, genotype variations that will increase the chance of survival and reproduction of some organism are preserved and will be inherited. But, the sources of this genotype variations are not changes in the environment, nor response to the stress, but the mutations in the genome of the organism. If a mutation is not lethal, it could remain in the population, increasing the variation is some traits.