The underlined word Those is a determiner which is the plural form of that'.
'Those' is a demonstrative determiner that is used to demonstrate the identity of the thing referenced by the following noun.
For example, Those mangoes are ripe.
As we have to identify which part of speech the word belongs to, we need to know the significance of the parts of speech.
Parts of Speech: A category to which a word is assigned in accordance with its syntactic functions.
In English, the main parts of speech are noun, pronoun, adjective, determiner, verb, adverb, preposition, conjunction, and interjection.
Here, the word ‘Those’ refers to 'boys'. So, ‘Those’ will be categorized as a determiner.