Cinder was indeed smart enough to know what 'betrothed' meant. He was on the inside completely shocked by this. He did not think a girl would have been interesting to him but now there was one made for him and hearing what La was saying though the proper romantic setting meaning that was mentioned eluded him. 
Though hearing Stella talk and then the others gossip he was unsure of what to say without flanning the flames, being inexperienced with girls on a social level outside of the classroom he would look at Kiri Falls with a slight awkward blush as he would not be used to this as a whole while feeling like he got literally blindsided from out of nowhere but at the same time he did not know exactly what to say to La right now as he struggled to have at least some thoughts on the matter without at least embarrassing himself.
[color=ed1c24]"I...I don't know what to really say..."[/color] He managed to get out though part of him wanted to address the gossip between La and him but felt like she did that already.
[color=ed1c24]"She seems mannered...and nice."[/color] shyly saying this as he would respond with a gentleman's bow to her.