Final answer:
The sentence 'Because Madelyn forgot to read her AP Lang assignment, she could not participate in the Socratic seminar, which gravely disappointed her classmates' is classified as a complex sentence because it has one main clause and two subordinate clauses.
Step-by-step explanation:
The sentence in question is: Because Madelyn forgot to read her AP Lang assignment, she could not participate in the Socratic seminar, which gravely disappointed her classmates.
Upon analysis, this sentence can be classified as a complex sentence. A complex sentence contains one main clause and at least one subordinate clause. The main clause here is 'she could not participate in the Socratic seminar,' which can stand alone as a complete sentence. The subordinate clause 'Because Madelyn forgot to read her AP Lang assignment' cannot stand alone and provides a reason for the main clause. Another subordinate clause is 'which gravely disappointed her classmates,' adding additional information about the effect of her not participating.