翻译：令父母担心的是，他决定不吃早餐了。(What)

what his parents worried about is he decided not to have breakfast.
Or:
what made his parents worried is ......
worry是不及物的，后面得有about.没有的都错。

What his parents worried is that he decided not to have breakfast.

What worried his parents is that he decided not to eat breakfast any more.

What worried his parents most was his deciding not to have breakfast any more.

What made his parents worried about is that his dicision that he would not to have breakfest.

What worried his parents was that he decided not to have his breakafast.