The correct answers are The driver went to the hospital to deliver pizzas. and The mayor visited the hospital to thank the nurses.

In standard English, when someone goes to a hospital as a visitor, worker, or delivery person (and not as a patient), we must use the definite article "the" to refer to the physical building.

(Fun fact: American English uses "the hospital" even for patients, while British English drops the article and says "in hospital" for patients! But for visitors and pizza drivers, both use "the".)