The correct answer is: Astonished.
To startle is to cause someone to feel surprise or fear. When someone is astonished, they are amazed or shocked. They may feel a sense of disbelief or wonder.
Amused is to feel or cause amusement. When someone is amused, they find something funny or entertaining. They may laugh or smile.
Relaxed is to be free from tension or anxiety. When someone is relaxed, they are calm and at ease. They may feel comfortable or at peace.
Endless is having or seeming to have no end. When something is endless, it goes on forever or for a very long time. It may seem like it will never end.
In the context of the question, the correct answer is astonished. This is because the word “startled” implies a feeling of surprise or fear. The other options do not convey this same meaning.