B. sounds better. The first option makes it seem as if the meeting has already occurred, and "was used" is correct if that is the intended meaning. "is being used" is best for if the meeting is presently occurring. "will be used" would be for a meeting later today.

Both would be possible to use, but "B" is more common. It does not necessarily mean the meeting is in progress, but that the room is reserved.

Thanks Mojave, I got it, so here in this question, future tense is preferred over present tense.

Mara gives a great answer. I just wanted to say that, in this context, "is being used ... today" and "will be used ... today" give almost identical "future" meanings.