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,
"<em>Neither schools nor banks are open on the third Monday in January </em><em>because</em><em> it is a national holiday honoring Dr. Martin Luther King, Jr</em>",
the "<em><u>Neither schools nor banks are open on the third Monday in January</u></em>" is the independent clause and "<em><u>it is a national holiday honoring Dr. Martin Luther King, Jr</u></em>" 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.