The correct option is C Whenever you need my help
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, 'whenever' means 'at whatever time'.
Option A is incorrect because it is an adverb clause of reason, not time.
Similarly, option B is incorrect because it
is an adverb clause of contrast, and not time.
Option D is incorrect because the answer lies in C.