Read the passage.
i am surprised that the change in your feelings should be so great.—he is, just what he ever was, only more evidently & uniformly devoted to you. this is all the difference.
in this passage from “on making an agreeable marriage,” what surprises jane austen about her niece’s feelings for mr. j. p.?

a) mr. j. p. started smothering fanny with devotion, so fanny’s feelings should have changed sooner.
b) mr. j. p.’s feelings haven’t changed much at all, so it isn’t clear why fanny’s have changed so much.
c) mr. j. p. started wearing his army uniform, so austen wonders why fanny’s feelings didn’t change more.
d) mr. j. p. started telling fanny his feelings, so austen wonders why fanny’s feelings didn’t change.

B). Mr. J. P.’s feelings haven’t changed much at all, so it isn’t clear why Fanny’s have changed so much.

Explanation:

'On Making an Agreeable Marriage' is one of Austen's numerous works that intends to highlight the materialistic nature of marriage(women were married for money(financial security) purpose merely) and argues for 'an agreeable marriage' that has love and affection which she considers the most significant thing in a marriage.

As per the question, the thing that surprises Austen about her niece's feelings for Mr. J.P. when she realizes that 'Mr. J.P.'s feelings remained unchanged yet why her niece has transformed so much' as she doubts how could she face such 'great changes' in her feelings suddenly and wants to ensure that whether she truly loves the man.

Austen thinks that her niece is too young to handle the huge responsibility like 'marriage' and thus, guides her marry the only one whom she loves as she doesn't want her niece to suffer like other women in society after having loveless marriages. Thus, she  Therefore, option B is the correct answer.

answer: do

the 'italicized phrase' is a noun clause that is used as a direct object.