Because the reader needs to be able to understand what he/she is talking about.
It seems that you have already filled the correct pronoun in the blank provided. Yes, the correct answer to complete this sentence is the pronoun "this". The pronoun "this" is an example of a demonstrative pronoun. Other demonstrative pronouns also include these, those, and that. Hope this answer helps.
Part A
Helen Keller's word choice in her description of her lessons with her teacher help to reveal that she is a very detail-oriented person. Being blind and deaf, she only had the sense of touch, smell, and taste, all of which are not very easy to receive information through. The way Helen Keller was able to describe the world around her was with the feelings and the smells of the earth, and this tells us how happy she is to pay her mind to these details. (I wrote this without knowing it was multiple choice, but yeah the answer is B.)
Part B
The word choice in the second and third paragraphs help to develop the idea presented in Part A? The answer is C.
The correct choice would be A. mower's.
When you say mowers, that would indicate that you are talking about more than one mower because it is in plural form. When you say mowers's, that would indicate that you are referring to more than one mower. Mowers's is incorrect because you say lawn mower and not lawn mowers, unless you are talking about more than one mower.