Step 1: Understanding the Question:
We need to determine the part of speech of the word "about" at the end of the sentence.
Step 2: Detailed Explanation:
Let us break down the sentence into its clauses to understand the function of "about".
"This is the book" is the primary independent clause.
"I was telling you about" is a relative clause that modifies the noun "book".
Normally, in standard declarative order, the sentence structure would be: "I was telling you about the book."
In this standard order, "about" clearly connects the action "telling" to its object "the book", showing the topic of the conversation.
Words that link verbs to nouns or pronouns to show relationships like direction, time, location, or topic are called prepositions.
In the given sentence, the object "the book" has been moved to the front of the sentence, and the relative pronoun (that/which) has been omitted.
Even though "about" is left stranded at the end of the sentence without its object immediately following it, its grammatical identity does not change.
This grammatical phenomenon is called a "dangling preposition" or a "stranded preposition".
It does not act as an adverb because it is not modifying the verb in isolation; it inherently relates the verb back to the noun "book".
Therefore, "about" remains a preposition.