I believe that the answer is; D. Move the comma and place it after softly.
The sentence would look like this:
Geoffrey mumbled softly, "She could have fixed it. I know she could have."
b. garden
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.
