The correct option is
C cry over spilt milk
In the given conversation: Gina is angry that she has missed her bus.
The expression, 'Do not cry over spilt milk' means 'Do not dwell pointlessly on past misfortunes'. Milk, once spilt, cannot be retrieved. This is an appropriate response to Gina because there is no point in getting angry or upset about the missed bus. The situation cannot be reversed. Hence, option C completes the conversation meaningfully and is correct.
The expression, 'Washing one's dirty linen in public' means, 'discussing one's personal affairs or problems in public'. Such a discussion would possibly invite ridicule or unpleasant comments from others and is avoidable. However, there is no personal affairs or problems being mentioned by Gina in the previous sentence. Hence, the expression in option A is incorrect.
The expression, 'Those who live in glass houses should not throw stones', means, 'Do not criticize others if you have similar weaknesses yourself'. In the first sentence, Gina is not criticising anybody. Hence, the expression in option B is not an appropriate response.
The expression, 'bark up the wrong tree' means 'to be attributing a wrong reason for something or following the wrong path to achieve something'. This expression is not relevant to the given conversation as Gina is not attributing any reason for something or following a wrong path. Hence, option D is incorrect.