Grammar Problem: So or This or That or It?

Here's a question in my English test paper that really puzzles me, who is a Chinese high school student. All you native English speakers out there looking at this thread, please help me decide which one sounds right:

" Amy lost her money and Mother wasn't surprised that she did _____ as she was too careless. "

Which one of the following words fits best in the blank?
A. That
B. It
C. This
D. So

"did so" is a very childish type of expression.

I personally think any of the words could be used.

Personally, I would rephrase "she did _____", with, "she had". I have also replaced 'too careless' with 'so careless'.

"Amy lost her money and Mother wasn't surprised that she had, as she was so careless.

Otherwise I think that 'this' is probably your best choice.

" Amy lost her money and Mother wasn't surprised that she did this as she was too careless. "

Thank you for the reply, Robin.
But why would you replace "too careless" with "so careless"?

I don't find "did so" childish. This is what I'd put there though any of these could do.

<<"did so" is a very childish type of expression.>>

No it certainly is not. "Did so" in a sentence like this is a rather formal expression, and there's nothing childish about it. In answer to Justin's question, I agree with Jim: of the possible answers that the test gives, I think "did so" would be the best.

I agree. "Did so" is the best choice. I don't see what's childish about it. In fact, it sounds like something a child wouldn't say.

This sentence sounds more natural:
"Amy's mother wasn't too surprised that Amy was careless enough to lose her money."

Justin,
I agree with almost everyone else in this thread. "So" is the most correct response. "So" is here an adverb that modifies the verb "did." But it may be helpful to think of it as a kind of pronoun that stands for the phrase "lost her money." We English speakers use "so" in this context to avoid redundancy. It would sound awkward, would it not, to say, "Amy lost her money and Mother wasn't surprised that she did lose her money, as she was too careless." It is a very common construction in English. Here are a few more examples to help you get the feel of it:

"Whenever his father told him to mow the lawn, he did so."

"Why are you angry. You asked me to drive you home and I did so."

And you must recognize that, as many have already pointed out, this "did so" construction is in no way childish. As a matter of fact, in the sentenence you provided it sounds quite formal. Robin seems to have for some inexplicable reason taken the "did so" completely out of context and reframed it into an argument between children. One child says, "Ah! You broke the lamp!" The other replies, "Did not!" Child one fires back: "Did so!" Child two: "Did not" Child one: "Did so!" etc. I make this assumption because it is the only way to understand Robin's apperantly nonsensical reply.

"Did so" is not childish at all.

Is so.