Answer:
D). Moving hardly a whisker.
Explanation:
As per the given excerpt, the words that would assist the readers to understand the meaning of 'serenely' as 'calmly or peacefully' would be 'moving hardly a whisker' as it implies that tomcat was moving so peacefully without 'making a sound' that it barely moved a whisker. This contextual evidence assists the readers to perceive the intended meaning of 'serenely' as implied by the author. Thus, option D is the correct answer.