53.One of the most important questions they had to consider

答：选C
解析：本题考查代词的用法.四个选项中,只有that可以代替句单数可数名词或不可数名词,以避免重复.如：The output of steel this year has increased by 20% compared with that in 1992.
今年钢产量比一九九二年增长了百分之二十.（此句中,后面的that指代前面的output这个不可数名词）.又如：The book on the desk is not so interesting as that in the desk.(此处的that指代前面的book这个单数可数名词,相当于the one).本题句意：他们必须考虑的最重要的问题之一就是公共卫生问题.如果还原这个句子,即：One of the most important questions they had to consider was the question of public health.（此处的the question是单数可数名词,因此可以用that或the one替代）,故选C.