Which of the following is true about natural selection?

Natural selection is a fundamental mechanism of evolution that operates on the variations present within a population. The statement that it acts on phenotypic variations among individuals is accurate because natural selection favors traits that provide some advantage to individuals in a given environment. These traits, or phenotypes, can influence an individual's ability to survive and reproduce. For example, a certain coloration or physical ability may help individuals avoid predators or find food more efficiently. Over time, those individuals with advantageous phenotypic traits are more likely to pass on their genes to the next generation, leading to changes in the population.

The other options do not accurately represent the principles of natural selection. It does not guarantee that all individuals will reproduce successfully, as only those better adapted to their environment tend to leave more descendants. Genetic diversity is crucial to the process, as it provides the raw material for natural selection to act upon; a homogeneous population may struggle to adapt to changing environmental conditions. Additionally, natural selection is not a process that occurs instantaneously; it typically takes place over many generations, as advantageous traits become more common through gradual changes within the population.