Final answer:
Mary isn't easily frightened like Martha.
Step-by-step explanation:
In the given excerpt from The Secret Garden, the best description of how Mary and Martha are different can be found in option C, which states that Mary isn't easily frightened like Martha. Throughout the story, it is evident that Mary is brave and fearless, while Martha tends to get scared more easily.
One example of Mary's fearlessness can be seen when she explores the secret garden on her own, without any hesitation or fear. On the other hand, Martha gets frightened by various things, such as the sound of the wind or the idea of ghosts.
Therefore, the correct answer is C. Mary isn't easily frightened like Martha.