When first used in medicine in the 1940s, penicillin was uniformly effective in killing the bacterium staphylococcus aureus. today, has led to an increase in antibiotic-resistant alleles, and humans are increasingly at risk from untreatable staphylococcus aureus infections

The correct answer is "natural selection in Staphylococcus aureus populations".

Explanation:

Natural selection is a process at which populations of organisms develop advantageous traits under the premise of survival of the fittest. In this case, the continuous exposure to penicillin and other antibiotics since 1940s have a natural selection effect in Staphylococcus aureus populations, allowing an increase in antibiotic-resistant alleles in the bacteria, which acts as an advantageous trait for them. Antibiotic resistance is a serious medical concern that has been addressed by a culture of responsible use of antibiotics and development of novel treatments for infections.

the answer is c on e2020, bud.