Final answer:
The assertion that would most support the author's statement is that invaders from Alexander the Great to the Romans tried to integrate Egypt into their empires.
Step-by-step explanation:
The assertion that would most support the author's statement that "Egyptian culture dwindled under the Classical world's onslaught" (paragraph 4) is C. Invaders from Alexander the Great to the Romans tried to integrate Egypt into their empires. This statement implies that Egypt was subject to foreign rule and control, which would likely have had a negative impact on Egyptian culture.