The correct option is B Since you have realized your mistake
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.)
From the options, "Since you have realized your mistake" is
an adverb clause of reason, 'since' is a subordinate conjunction that shows reason, hence
option A is correct as it's the most suitable option.
Option B is incorrect because it is a phrase.
Option C is incorrect because 'since' is only a word, not a clause.
Option D is incorrect because the answer lies in A.