it was in1990, my senior year in high school,___I realized what kind of man this boss was.

这是一个强调句．强调句是it is ……that／who……
在这里当然是用that了，who表示人
强调句和其他句子的区别是强调句去掉it is ……that／who……之后仍然是个完整的句子。象这句话，去掉之后是 in1990, my senior year in high school,I realized what kind of man this boss was. 句子很完整。