Final answer:
The nurse explains that antibiotics are only effective against bacterial infections, and since croup is caused by a viral infection, antibiotics would not be effective in treating it.
Step-by-step explanation:
The nurse makes the response b) Antibiotics are only effective against bacterial infections.
Antibiotics are drugs specifically designed to kill or inhibit the growth of bacteria. They do not have any effect on viruses, which are the cause of croup.
Therefore, since croup is caused by a viral infection, antibiotics would not be effective in treating it.