The correct option is
B cross that bridge when you come to it
In the given conversation, the son expresses his worry about an imagined future. He wonders how he will manage in case he is not able to make good friends in the new school which he is going to join in future. Such a scenario may or may not happen. However, the son is worrying beforehand.
The expression, 'cross the bridge when you come
to it' means 'face difficulties as they happen and not worry unnecessarily about them beforehand'. Given this meaning, this is an appropriate expression which can be given as a reply by the father. This is because the father advises the son to work on his exams at that point in time. And not to worry about the future problems, which have so far not arisen. Hence, option B is the correct answer. Option A: The expression, 'to vent one's spleen' means, 'to get rid of one's feelings of anger caused by someone or something by attacking someone or something else'. In the given conversation, the boy is not angry. He is worried. Hence, this expression cannot be an appropriate response by the father and option A is incorrect.
Option C: The expression, 'to put your foot in your mouth' means 'to say or do something that you should not have, especially something that embarrasses someone else'. In the given conversation, the boy is expressing his worry. The father cannot, by way of response, ask the son to do something that embarrasses him. This expression is out of context and option C is incorrect.
Option D: The expression, 'to change horses in midstream means 'to make major changes in an activity that has already begun'. In the given conversation, the son is not talking about changing an activity. He is only expressing his worry. Hence, this expression cannot be an appropriate response by the father and option D is incorrect.