Imagine two people are talking about something privately and someone else just passes through the way they are staying and unwantedly, hears an important issue from among their' words, but does not show that he / she heard it and goes away. One of them says:
- I know that he / she heard what you said, but he / she didn’t....................it.
which choice would be a more natural one in this self-made sentence of mine?
d) let on it / let it on