Khushi said "kaise insaan hai aap?" and Arnav looks flustered for a millisecond before replying "jaisa bhi hu, tumse behtar hu.. kamse kam apne aap se toh jhoot nahi bolta.." (then the music goes off into silence while he says) "sach bolne ka, achcha banne ka naatak toh nahi karta.." and then that music starts which is usually meant for their moments.. like when in the suicide suicide scene, after she's done and just before he leaves, that music starts.. also, dupatta laptop scene, as he starts to remove the dupatta, that music starts.. so, I think the fact that that music plays here instead of any other hateful music, I feel, itself means that this was a moment.
Also, after he says that, he gives her a delicious smirk + almost-a-minuscule-beginnings-of-a-smile expression with the most smoldering look. I'm adding a pic of that look that almost says "I know you want me, you know I want you" (not exactly Flo Rida music, but you get the picture :-p).

You'll also notice that Khushi looks mighty flustered after that. And why would she say something like "aap baat ko kahaa se kahaa lekar aa gaye?" instead of just looking hurt or something (like she always does when he accuses her). In fact, she didn't look hurt, just really flustered like damn, does he know how I feel types. And why would say something on the lines of "why are you bringing this (what's between us) up when we're talking about Lavanya and you?"
So, I'm wondering if this is what it all means: Arnav finally decided to grow some balls and throw it out there that he acknowledges the attraction between them. Unlike her, who, in spite of being attracted to him herself, is lying to herself (by not acknowledging the very evident pull between them) and trying to be miss goody two shoes by pushing Lavanya's case. In that way, apne aap se jhoot bol rahi hai (that she feels nothing for him) aur sach bolne ka aur achcha banne ke naatak kar rahi hai (by pushing Lavanya's case and hence trying to show herself as nice, and of course the letter itself is a lie too).
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