The correct answer that combines the given sentences using a subordinate conjunction is C.
C. As Jacob worked hard and wrote a beautiful melody, Jill wrote the words to make it a song.
In this sentence, "as" is a subordinate conjunction that shows the relationship between the two clauses. The first clause, "As Jacob worked hard and wrote a beautiful melody," sets the scene and provides context for the second clause, "Jill wrote the words to make it a song." The use of the subordinate conjunction "as" indicates that the second clause is dependent on the first clause, and that the two clauses are connected in terms of cause and effect or time.
Option A is incorrect because the subordinate conjunction "so" indicates a cause-and-effect relationship that is not present in the original sentences.
Option B is incorrect because the coordinating conjunction "and" does not create a subordinate relationship between the two clauses. Instead, it connects the two clauses as equal and independent ideas.