One of the most important questions they had to consider was of public health. A. whatB.

问题补充：

One of the most important questions they had to consider was of public health.

A. what

B. this

C. that

D. which

答案：

A. what

B. this

(答案→)C. that

D. which

解析：考查代词。分析句子结构可看出they had to consider是定语从句，可见was是系动词，根据选项可判断后面不是句子，不需要疑问词，排除A和D。通过句子结构的分析空格应是指上文的question这个名词。语法告诉我们that指上文，而this指下文。句意是“他们不得不考虑的最重要的问题之一就是公众健康的问题”。