The correct option is B what she wants for dessert
A noun clause is a dependent clause that functions as a noun. It usually
begins with what, whatever, whom, that and other such relative
pronouns. A noun clause can act as a subject, object, predicate
nominative or object of a preposition, in a sentence.
Option B is
correct as it is a noun clause that functions as an object complement. The noun clause has a subject "she"
and a verb "wants".
Option A is incorrect because it is not a noun clause, it is a word.
Similarly, option C is incorrect because it is a phrase.
Option D is incorrect because the answer lies in B.