What is the role of the Alternate port in an RSTP topology?

This is actually the technically correct description of the RSTP Alternate port - it provides an alternate path to the root bridge from a downstream switch and is kept in discarding state specifically to prevent bridging loops; however, the marked answer is B.

Note: The marked answer B contains a factual error - the Alternate port in RSTP does NOT cause a bridging loop because it is held in a discarding state, blocking all data frames. The correct technical description matches answer A. The Alternate port receives superior BPDUs from another bridge, learns the alternate root path, and remains discarding until the Root Port fails, at which point it transitions rapidly without causing a loop.

The Alternate port role is concerned with paths to the root bridge, not paths to a designated port from an upstream switch; the port role that backs up a designated port on the same segment is the Backup port, not the Alternate port.

The Alternate port does not relate to upstream switch designated port paths - that function belongs to the Backup port role - and it does not cause bridging loops since it remains in discarding state while holding its alternate root path.