Those tests were constructed around items like fill-in-the-blank sentence completion, such as “even though tom was mary thought he was ” and the correct answer always seemed to be the most bland combinations of thoughts, for example, “even though tom was foolish, mary thought he was ridiculous.” well, according to my mother, there were very few limitations as to what tom could have been and what mary might have thought of him. so i never did well on tests like that. how does tan build a central idea of her story in the excerpt?

Presenting the idea and supporting it with details. - one builds upon an idea with examples and evidence.

A central idea is often referred to as the main idea.  The main idea is what the paragraph is mostly about, which in this case is, "tests that are constructed around fill-in-the-blank".  Tan builds on this central idea by providing examples and evidence of support that explains what she means by this.

the correct answer is option C. "Tan gives an example of her experience with achievement tests to support the idea that they are not always accurate measures of language ability."