Final answer:
The interjection 'Yikes' fits the context of the sentence as it indicates a subtle, alarmed reaction, suitable for a stealthy situation.
Step-by-step explanation:
The interjection that completes the sentence "Rita whispered as she and Zander crept silently into the garage." could be any word that expresses a sudden emotion or sentiment. In this context, (b) Yikes would be an appropriate interjection because it suggests a subtle reaction to a potentially surprising or alarming situation, which is fitting given the stealthy actions described.