The correct answer is D. Between
Step-by-step explanation:
An infinitive is the basic form of a verb compound by the preposition "to" and the bare verb, for example, to eat, to run, to go or to cook. In the case of a split infinitive a word or groups of words is inserted between the preposition "to" and the bare infinitive which is usually an adverb, for example, "she was advised to slowly run" in which "to run" has been separated or split by the adverb "slowly". Thus, a split occurs if one or more words are inserted between the two parts of an infinitive.