The correctly punctuated complex sentence is:
"The dean was disappointed because the study showed a drop in enrollment."
A complex sentence consists of an independent clause (a complete thought that can stand alone as a sentence) and one or more dependent clauses (incomplete thoughts that cannot stand alone).
These clauses are joined by subordinating conjunctions or relative pronouns. The structure allows for the expression of more intricate relationships between ideas, offering a deeper level of meaning and complexity in writing.
Complex sentences enhance the variety and depth of communication by combining different elements within a single sentence.