It is important to introduce children to healthy nutritious foods from they begin to eat

问题补充：

It is important to introduce children to healthy nutritious foods from they begin to eat solids.

A. what

B. when

C. that

D. which

答案：

A. what

(答案→)B. when

C. that

D. which

解析：B考查名词性从句。句意：从孩子可以吃固体食物开始就引导他们食用健康营养的食物是很重要的。when在此引导宾语从句，作介词from的宾语，且在从句中作时间状语。