Which one of the following socket API functions converts an unconnected active TCP socket into a passive socket?

In standard BSD socket programming:

• By default, a socket created with socket() is an active socket (assumed to initiate connections via connect()).

• The listen() function transitions an unconnected active socket into a passive socket, indicating to the OS kernel that it should accept incoming connection requests directed to this socket.

• bind() assigns a local protocol address (IP and port) to the socket.

• connect() establishes an active TCP connection.

• accept() retrieves the next completed connection from the queue of a listening (passive) socket.

Therefore, option (C) is correct.