Answer:
"The reason I am late" is the noun clause, and it is being used as a subject.
Step-by-step explanation:
While no words are italicized here, I know this to be a noun clause because a clause contains a subject and a verb. So if all of these words are italicized, it is a subject. If "I am late" is italicized, it is a subject complement. Both of these contain subject and verbs, so without the italics, these should give you the answer.