Question

A pathogenic strain of bacteria has become resistant to an antibiotic that once could kill it. What has happened to these bacteria to make this happen?
-Most of the bacteria make changes to their DNA to protect themselves, making themselves immune to future attacks.
-Only the few bacteria that were immune to the antibiotic survived and reproduced, making all their offspring immune to it as well.
-Some of the bacterial cells were able to produce toxins against the antibiotics, making them more likely to survive.
-The bacterial cells learned to remove the receptors on their cell surfaces.

Answer 1

The answer is -Only the few bacteria that were immune to the antibiotic survived and reproduced, making all their offspring immune to it as well.

Bacteria (or any other organism) are not able to make changes to their DNA in order to protect themselves or to learn to remove the receptors on their cell surfaces. If they were able to produce toxins against the antibiotics, they would all survive.
These leave the second choice as the correct answer. This is the real situation, and is a good example of natural selection.