This argument can be broken up into:
Premise) Without a college degree or higher, citizens do not have access to the highest quartile of salaries
Premise) only individuals earning salaries in this highest quartile have enough disposable income to buy real estate and invest in long term assets
Conclusion) higher education is essential for achieving prosperity
On assumption questions, it is typically better to develop a solid understanding of the argument like this and then head to the answer choices without trying to predict the correct answer. Here I could see a pretty glaring error here though in its narrow definition of achieving prosperity even before I moved to the choices.
Answer choice A: This definitely does not have to be true. Maybe college degrees are just a vetting process? Also, the answer choice is very strong ("all jobs" "always covered")
Answer choice B: Definitely don't need to assume this. Maybe some people invested in long term assets but not real estate. Maybe they bought real estate other than their own house.
Answer choice C: This nails it. Negate it and plug it back into the argument, and you can see the argument gets crushed, and therefore this is a necessary assumption. Negated, this assumption would read: Prosperity does NOT have to include ownership of real estate or long term assets. If that is the case, then this argument becomes pretty silly, as the whole logic is based on achieving prosperity through having the ability to own real estate or invest in long term assets.
Answer choice D: Understanding what prosperity is has nothing to do with this argument. Maybe a lot of people who did not go to college and have not achieved it still understand it. This argument could still hold, and therefore this is not a required assumption.
Answer choice E: Irrelevant output question that is focused on the wrong details. We don't have to assume this; what if a bunch of people who did not go to college were given real estate as a gift or something? We already know that people without a college degree or higher cannot afford to buy real estate or long-term assets, so answer choice E is just doing a bad job at trying to confirm a premise that we already know, and is therefore not a required assumption.
I hope this helps!