Which is the correct description of a pinned buffer in the database buffer cache?

Which is the correct description of a pinned buffer in the database buffer cache?

A.
The buffer is currently being accessed

B.
The buffer is empty and has not been used

C.
The contents of the buffer have changed and must be flushed to the disk by the DBWn process

D.
The buffer is a candidate for immediate aging out and its contents are synchronized with the block contents on the disk