If one person is asking to use the conference room, they are likely asking if it is free. This means that there is nothing else scheduled for it. Based on the short conversation here, it seems like the person who is talking wants to use the conference room right now, so they are asking whether it is available.
The responder confirms that they can use it. In the blank, several different words can be used, including
is free, is available, is not booked, is accessible, is not busy, is unoccupied, is handy, etc.
Any of these verb phrases would be appropriate for the blank.
