A preposition is found in a sentence in which it introduces information to the reader like a location perhaps. The object of the preposition can be a noun, a pronoun or even a gerund that is usually together with the preposition in order to complete a prepositional phrase. In the statement above, the prepositional phrase is "on the garden", therefore the preposition is "on" and the object of the preposition is the "garden" to which the preposition "on" refers to.
Let me start with why it isn't C. C is true but it's not more important than what A says. A talks about how Martin and Kennedy agree with their words which is what the question asks.
It effects the reader because the tone in the story can effect their emotions and certain words can have a different effect. The reader becomes more upset and negative when reading about negative things. The reader would feel more positive if the book talked about more positive things because of the same reasons it would effect someone in a negative way; certain tones and words
