Hi Prakruti, what we (my spouse and I) did was to pick conversations that shows the flow of time. We have at least 3 years of conversation, so we picked something from 2014, 2016, 2017. (The gaps are there because we have photo proofs and a timeline of what we did for those times.)
As for what kinds of conversation, it could be something as simple as "I'm going to pick you up for dinner" or "what should we eat for lunch". Of course, if it helps your case, it could be something big like wedding planning, trip planning...etc. I believe, as what I've read on the forums is that they just want to know you have been in contact with your sponsor throughout the relationship. We Skype video a lot, so I prepared pages of Skype history showing that we've made calls to each other.
We also included the context of the chat (sometimes it's not obvious what we're talking about) on a separate piece of paper to help the officer understand. Our take is that, if we show these conversations to other people, would they come to the same understanding that we have a strong relationship going on in light of other evidence present.
I hope this helps!