Answer: Option B
:
B
The last line of the third paragraph "Given that we have seen that Kant defends himself against the charge of idealism, it is worth returning back to that argument more critically."
Also the fifth Paragraph states that "The external world, in the Kantian framework, is ultimately the construct of our faculties of mind".
So adding these two lines leads to option (b).
The author himself states why he is critical about Kant’s arguments in the above lines. All the other options correspond to his contemplations and the author doesn’t link them to critical argument (which the whole passage is all about).