1. Simmi - *less possibility , but given the way the Ishra scene convo and it was overheard, and ishita was instructing Simmi to call, so she might have entered and overheard. But why do I feel it is Simmi as Ashok said that his informer can't do anything.
2 . next to come in number is Sarika, as discussed by many of members here, the same reason, as she would have killed rinki and Ashok could know this, so could be.
3.the one could be Romi and he is having a reason that after sarika job he is feeling insecure as everyone now and then taunt him, so he can think that since Raman is caring only for his home, his due he will never get as being a younger than him. So he may have got inticed by Ashok and would have come into words and can also see his own profit. So possible largely.
So whom do you think. Mihir could also be a possibility as no one ever doubted him and he could be the best player.