It calls education one of the most important investments a c

本句是定语从句.在investments和a之间省略了关系代词that.因为先行词investments做了make的宾语了,可以省略that.又先行词被形容词的最高级所修饰,所以关系代词只能用that了.主句是：It calls education one of the most important investments .从句或者子句是a country can make in its people..用2个完整的句子来表示就很明显了.
1.It calls education one of the most important investments .
2.A country can make investments in its people..