The correct answer is D. Has been studying
Explanation:
Determining the correct verb form is not always easy as it is necessary to consider both the tense which is the time in which the action takes place (present, future or past) and whether the verb is grammatically correct. In the case of the tense, it is important to identify contextual clues including words such as yesterday or next year that show the action belongs to a certain tense. While in the case of grammar it is important to consider there is an agreement between the subject and the verb which means a singular subject can only be followed by a singular verb form.
In the sentence "A team of explorers ______ the ocean floor" there are no contextual clues that show the verb tense should belong to a certain time, which means any tense is acceptable as long as there is a subject-verb agreement. In this case, the subject of the sentence is "team" this type of subject is called a collective subject because it defines a group of people but shows them as a unit. This means even though we know the group is composed of different explorers the subject is singular as it refers to a unit or one team of explorers instead of multiple teams. Because of this, this sentence should have a verb form that also indicates singular which means the best correct form for completing the sentence is "has been studying" as this is the only option that has a singular form and can be together with the singular noun "a team of explorers".
