The correct option is B I dance.
In the most simple words, a clause is a group of words made up of a
subject and a verb. These are essentials in a clause; without the
subject and verb the group of words can't be called a clause. (Unless it
is a part of a larger sentence that contains the same subject/verb).
A clause is either dependent or independent: independent clauses can stand alone as sentences.
Option B is correct because it is a clause, complete with the subject "I" and the verb "dance". Verbs like "Dance" do not require an object.
Option A is incorrect because these words do not include a subject nor a verb; hence it isn't a clause. ('Buying' is not a verb, it is a participle which requires a helping verb.)
Option C is incorrect because it does not contain a subject.
Option D is incorrect because it does not contain a verb; 'after' is an adverb and 'she' is a subject pronoun.