To specifically answer your multiple choice question, the best answer would be "E," all of the above. Atticus has previously warned Jem not to kill mockingbirds, and Miss Maudie further explains to Scout that they are harmless creatures who cause humans little or no problems, put on this earth to "sing their hearts out for us."
"They don't eat up eat up people's gardens, don't nest in corncribs..." (Chapter 10)
Symbolically, the mockingbird also represents several of the innocent, harmless humans in the story. Most of the children can be considered as mockingbirds as well as adults such as Boo Radley and Tom Robinson, both of whom have been accused of crimes they did not commit.