This cartoon jokes about the formation of superbugs; this is an example of natural selection, due to

a
the more bacteria come into contact with antibiotics, the more chances they have to develop a population that is resistant to the drugs.
b
bacteria are extremely aggressive to other bacteria since they often form street gangs against difference populations.
c
all bacteria naturally have resistance to antibiotics, regardless of population size.
d
the less bacteria come into contact with antibiotics, the more chances they have to develop a population that is resistant to the drugs.