In the given excerpt, the word "tremulous" is used to describe Elnora's voice. This term suggests that her voice was shaky, quivering, or characterized by nervousness or fear. When looking for a word that can replace "tremulous" without changing its meaning, the most suitable option would be:
D. nervous
This word captures the essence of a voice that is not steady or calm, similar to the implication of "tremulous." Choosing "nervous" maintains the idea of Elnora's voice conveying a sense of unease or anxiety in the conversation with the Bird Woman.