The correct answer is: B. under
The preposition “under” is used to indicate a location that is below or covered by something else. In this case, the tree is providing shade, which is a covering that protects from the sun. Therefore, the correct preposition to use is “under.”
The other options are incorrect because they do not accurately describe the location of the people in the sentence. The preposition “in” is used to indicate a location that is inside something. The preposition “ourselves” is used to refer to oneself. The preposition “ourselves in” is not a grammatically correct phrase.