Answer:
The correct answer is B. Pensively.
Step-by-step explanation:
The question refers to the relationship between two words with a similar meaning, where "accidentally" is related to "carelessly", since every accident is produced by lack of care, with which the accidental arises from carelessness.
Thus, the word "thoughtfully" is directly related to "pensively", insofar as they refer both to action through reflection and evaluative thinking of the decisions to be made by the person.