The round structure which is usually topped with a dome is best-known as a although it can also be referred to as a library.
a. tholos
c. rotunda
b. corbel dome
d. onion dome

The round structure which is usually topped with a dome is best-known as a rotunda, although it can also be referred to as a library. Correct C

The Pantheon erected at Rome about AD 124 is an example of a Classical Roman rotunda. Sometimes the term rotunda is also used in reference to the dome itself, whether or not the area beneath it is a single oval or circular room.