Answer:
The subordinating conjunction in the sentence is "because".
Explanation:
Subordinating conjunctions are those parts of speech that joins two clauses together, with one clause being a dependent clause and the the other an independent clause. In the provided sentence,
"Neither schools nor banks are open on the third Monday in January because it is a national holiday honoring Dr. Martin Luther King, Jr",
the "Neither schools nor banks are open on the third Monday in January" is the independent clause and "it is a national holiday honoring Dr. Martin Luther King, Jr" is a dependent clause. And joining these two clauses is "because". Thus, it is the subordinating conjunction as it joins the two clauses- dependent and independent clauses.
