Identify the part of speech that has been underlined:
He told us all about the battle.
(a) Adverb
(b) Preposition
(c) Conjunction
(d) Interjection

A preposition is a word used with a noun or pronoun to show how the person or thing denoted by the noun or pronoun stands concerning something else. The underlined word is 'about' is a preposition which means - on the subject of (something). For example, They always keep talking about us. In the given question, we have to find out if the underlined word is which part of the speech.
We will look at each option carefully and try to find out the meaning and figure out the answer through it:
Option a-adverb is a word that adds more information about a place, time, manner, cause or degree to a verb, an adjective, a phrase, or another adverb. For eg. He swims well.
Option b-preposition which is a word or phrase that is used before a noun or pronoun to show place, time, direction, etc. Some examples of prepositions are words like “in”, “at“, “on”, “of”, and “to”.
Option c-conjunction which is a word that is used for joining other words, phrases or sentences. For eg. She bought a shirt and a book.
Option d-interjection which is a word or phrase that is used to express surprise, pain, pleasure, etc.( for example: oh! Hurray! Or Wow!). For eg. Bah! That was a total waste of time. After analysing each option carefully, we came to the point that option 'b' is the right answer. 

So, the part of speech used in the sentence is a “preposition”.

Note: Parts of speech is a category to which a word is assigned following its syntactic functions. In English, the main parts of speech are noun, pronoun, adjective, determiner, verb, adverb, preposition, conjunction, and interjection.