Why is the answer choice (d) and not choice (a)
btw this is from the kaplan practice new sat book

Read the excerpt from "mother tongue." lately, i’ve been giving more thought to the kind of english my mother speaks. like others, i have described it to people as “broken” or “fractured” english. but i wince when i say that. it has always bothered me that i can think of no other way to describe it other than “broken,” as if it were damaged and needed to be fixed, as if it lacked a certain wholeness and soundness. what best supports the inference that tan believes nonstandard english is no less valid than standard english? tan spends a lot of time thinking about her mother’s “fractured” english. tan has trouble thinking of descriptive words when she is writing. tan’s american education makes it difficult for her to understand her mother. tan winces when she describes her mother’s english as “broken.”