Strains of the bacterium Staphylococcus aureus have developed which are resistant to several types of antibiotic. Scientists have named these strains MRSA. These strains have developed as a result of...?

The answer is natural selection.  you have a colony of the bacterium and one bacterium develops a mutation which allows it to become resistant to some types of antibiotics. the bacteria that dont have the mutation get destroyed by the bacteria and those that do survive. This is an example of natural selection as it is basically the survival of the fittest.