Why are cortisol analogues such as prednisone used for treating asthma?

Solution:

prednisone, are primarily used for their potent anti-inflammatory effects in disorders of many organ systems it works as  it is absorbed in the body, unlike inhaled steroids (anti-inflammatory asthma inhalers) that go straight to the lungs. ... Prednisone and other systemic steroids may be used to treat asthma attacks and help people gain better asthma control.

the answer to your question is:

c. complementary base-pairing

substitution is the answer in this case