For any strengthen question, (and weaken), the strengthening (or weakening) can impact any portion of the passage. It is usually the main conclusion, but could be strengthening a sub conclusion or premise. However, it is not always fully discernible during a timed section which portion is being impacted directly. It is perhaps more realistic to focus on the conclusion and how it was reached as a whole, and then examine which answers have either a positive, neutral, or negative impact on the argument. So, in short, it is possible to strengthen the sub conclusion but it isn't necessarily something we should focusing on explicitly.
Additionally, this question requires us to find the assumption. For the argument as a whole, it assumes that the parents will make their choice based on academics. This is an easier, more natural way to think about it, in my opinion.
I hope this helps. Please let me know if you have any other questions.
