Identify the part of speech the underlined word belongs to.
The spread of the fire was within the building.

The underlined word 'within' is a preposition that means 'inside (something)'.
For example: Play within the building.
Here, the word ‘within’ shows the relationship between ‘the fire’ and ‘building’ i.e. it shows where the fire occurred.
So, within will be categorized as a preposition.