答：選C
解析：本題考查代詞的用法.四個(gè)選項(xiàng)中,只有that可以代替句單數(shù)可數(shù)名詞或不可數(shù)名詞,以避免重復(fù).如：The output of steel this year has increased by 20% compared with that in 1992.
今年鋼產(chǎn)量比一九九二年增長了百分之二十.（此句中,后面的that指代前面的output這個(gè)不可數(shù)名詞）.又如：The book on the desk is not so interesting as that in the desk.(此處的that指代前面的book這個(gè)單數(shù)可數(shù)名詞,相當(dāng)于the one).本題句意：他們必須考慮的最重要的問題之一就是公共衛(wèi)生問題.如果還原這個(gè)句子,即：One of the most important questions they had to consider was the question of public health.（此處的the question是單數(shù)可數(shù)名詞,因此可以用that或the one替代）,故選C.