The question is that we should make it known __everybody how serious the population problem is.
问题描述：
The question is that we should make it known __everybody how serious the population problem is.
填to 还是by ,原因，
答
to
本句意为：问题是我们应该让大家知道人口问题是多么严重.
The question is （that we should make {it} known to everybody [how serious the population problem is].）
括号里面的是表语从句,大括号里的it是形式宾语,真正的宾语是中括号里的宾语从句.