Which of the following is a sentence fragment?
a. checking his mail each day was always fun.
b. knowing he would be late, she took a nap.
c. realizing you are getting older is sobering.
d. hiding in the tree fort he had built as a kid.

D. would be a sentence fragment because it dose not have a person or subject who is completing the action...My teacher explains it as " If some one just came up to you and said 'Hiding in the tree fort he had built as a kid.' would you understand them? You would be so confused but if it had a subject 'He was hiding in the tree fort he had built as a kid.' It would still be strange but you would understand what they were saying."

You didn’t include the multiple choice/ options list