Answer: Dependent Clause
Step-by-step explanation:
A dependent clause, also known as a subordinate clause, is a clause that cannot stand alone as a sentence because it does not express a complete thought. In the sentence "Freddy left his closet door open because", "because" is a conjunction that connects the dependent clause "Freddy left his closet door open" to an independent clause (the independent clause is not provided in this sentence so it is not possible to know what it is).
A dependent clause starts with a dependent word such as "because, since, after, although, while, when, where, if, whether, before, and even though" and it will not make sense standing alone. It always need a independent clause to complete the meaning.