The correct option is A where no one else can find it
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 place, "where" refers to place and it has the subject "no one else" and verb "find".
Option A is incorrect because it is a phrase, not a clause.
Option B is incorrect because it is not an adverb clause of place, "because" refers to cause.
Option D is incorrect because the answer lies in C.