Answer:
D). Discovery.
Step-by-step explanation:
The tone is demonstrated as the author's approach or attitude towards a specific subject matter which is clearly reflected through his/her word-choice and use of language. It primarily functions to provide the readers with a perspective to view the text and shape their responses or experiences.
In the given excerpt, the word 'discovery' most aptly describes its tone as it aims to display what the author found at the scene('landscape features of the domain-upon the bleak walls', 'vacant eye-like windows', 'a few rank sedges', etc.) and reveals his feelings of 'utter depression.' He is unable to analyze that 'what it was that unnerved him' and concludes that 'it was an insoluble mystery that was due to the distinct combinations and arrangements of the natural objects that have affected him. Thus, option D is the correct answer.