Answer:
'So' - Coordinating conjunction.
Step-by-step explanation:
'So' is a conjunction that can be used as both, subordinating, as well as, coordinating conjunction. As a subordinating conjunction, 'so' functions to join two clauses of unequal significance(a dependent clause and an independent clause) to convey the sense/meaning of 'so that'(show purpose). While as a coordinating conjunction, it is employed to link two equal and independent clauses to convey the sense of 'therefore.'
Since the given sentence conveys a sense of 'therefore' as the latter action('being unable to join swim team') was a consequence of the first one('decision of joining volleyball'), 'so' is a coordinating conjunction. Had it shown the purpose in the sense of 'so that', it would have been a subordinating conjunction.