The correct option is C since I started working here
An adverb clause is a group of words containing a subject and a verb,
and it functions as an adverb, by modifying an adjective, verb or an
adverb. Adverb clauses generally begin with a subordinate conjunction
(since, because, while, etc.).
Option C is correct because it is an
adverb clause of time, 'since' is a subordinate conjunction that refers to time.
Option A is incorrect because it is a phrase, not a clause.
Option B is incorrect because it
is a coordinate clause that refers to contrast whereas an adverb clause is subordinate/dependent.
Option D is incorrect because the answer lies in C.