Answer:
The answer is D: she had been speaking about Mr Elliot to others in the room.
There is no indication in the text that Anne had been speaking about Mr Elliot to others in the room. The other reasons listed suggest that Anne is familiar with Mr Elliot and perhaps has some kind of relationship or history with him, such as knowledge of his travel plans, discomfort at Mary spotting him, sensitivity to what the other ladies might know, and having met him in Lyme.