The conjunctions are used to link clauses to form meaningful sentences. In the above case, “but” is linking clauses in the sentences so that, it is considered as conjunction.
The little girl wanted a cookie is a spare sentence that wa slinked to “she didn’t take one” by a conjunction “but”.
Thus, the options (a), (c) and (d) are incorrect.
The little girl wanted a cookie is a spare sentence that wa slinked to “she didn’t take one” by a conjunction “but”.
Therefore, the correct option is – b. but.