I believe the correct answer is B.
A subordinating conjunction is a conjunction that introduces a subordinate clause, that is a clause that cannot be independent, but needs a main clause to form a well-formed phrase.
So, for example, "<span>Although he was embarrassed" is not an independent clause: it can't stand on its own. Here "although" is the subordinating conjunction, and the correct answer is:
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<span><span>A. Sentence 3- Although he was embarrassed, Jack called his mother</span>
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