Can you tell me the reasaon () your breing late for class this morning?

B
the reason for sth／doing sth：做某事的理由，原因。
of只用在固定搭配 by the reason of ：意为由于……

of

选择B.介词for表示原因，of表示所属关系

Bthe reason for sth做某事的理由，原因。