Susan sits . . . . . . . the tree. A. under B. up C. her D. she

The correct answer is A. Susan sits under the tree.

The other options are incorrect because:

• B. Susan sits up the tree is incorrect because it implies that Susan is sitting on top of the tree.
• C. Susan sits her tree is incorrect because it implies that Susan is sitting on her own tree.
• D. Susan sits she is incorrect because it is a grammatically incorrect sentence.

The word “under” is used to describe the position of something that is located below or beneath something else. In this case, Susan is sitting below the tree.