B. The mockingbird symbolizes the softer side of nature, because unlike most wild creatures, it doesn't harm human habitat.
Explanation:
Symbols help us connect ideas from a text with previous real life knowledge or things that most likely represent the same to a big audience.
On the excerpt, mockingbirds are represented as innocent creatures that 'don't do anything but to bring joy'. As said on answer B, mockingbirds as nature doesn't harm us, but only is in charge of give resources in order for us to have a joyful life.
