Hello there
Let's talk about a little bit of Morphology (a field of linguistics that study words and their composition)
Word: symphony
Morphology: syn + phony. It comes from the old latin: symphonia. In English the world only has 3 syllables.
Prefix sym
The English prefixes syn- along with its variant sym-, derived from Greek, mean “together.”
Therefore, correct answer is C: together.
Hope this helps