Under the Illinois Insurance Code, an insurer that has obtained a certificate of authority from the Director of Insurance to transact insurance business in Illinois is best described as:

Why this is the answer

Section 121 of the Illinois Insurance Code requires any entity transacting insurance in Illinois to first obtain a certificate of authority from the Director. An insurer that holds this certificate is referred to as 'authorized' or 'admitted.' Insurers without such authority are 'unauthorized' and are generally prohibited from soliciting business in Illinois, except through compliant surplus lines channels under Article V½. Surplus lines insurers are technically unauthorized but lawfully accessed through licensed surplus lines brokers; risk retention groups operate under separate federal-state authority.