The noun in this sentence which is uncountable, meaning that it names something that cannot be counted is the noun water.
You cannot count how many waters there are - it is uncountable. The other nouns, students and pitcher are countable, so they cannot be the correct answers, but rather water is.
The answer should be Judge Taylor hoped Atticus would defend Tom well because Judge Taylor knew that Atticus was the only lawyer that would explain this case in a way that would be heard instead of it being dismissed immediately.
I think the first quote best supports the theme "the power of language"