Step 1: Understanding the Question:
The task is to identify the part of speech of the word "Such" as it functions at the beginning of the given sentence.
Step 2: Detailed Explanation:
Let us analyze the structural syntax of the sentence.
The main linking verb in the sentence is "was".
In English grammar, every verb must have a subject, and the subject of a sentence must be a noun, a pronoun, or a noun phrase.
The word positioned as the subject of the verb "was" is "Such".
Here, "Such" means "of this kind" or "of that extreme degree". It is taking the place of a longer, descriptive noun phrase (e.g., "The nature of his ambition was...").
When the word "such" is used entirely alone as a subject or an object to represent a specific quality or thing, it functions as a demonstrative pronoun.
Let us evaluate the incorrect options:
(A) noun: "Such" is not a standard naming word.
(C) verb: It shows no action or state of being.
(D) adjective: "Such" can frequently be an adjective when it is placed directly before a noun (e.g., "Such ambition is dangerous"). But here it stands alone without a trailing noun, making it a pronoun.
(E) preposition: It does not show spatial or temporal relationships between nouns.
Therefore, it correctly functions as a pronoun.