There is no verb "to be" or "is" in the Greek phrase that you are asked to translate here. (So without a verb, it's not a full sentence, despite the initial capital letter and the full stop at the end.)
Greek isn't like Russian or Hungarian or Turkish or other languages where you can drop the verb "to be".
"This is the boy" would be Αυτό είναι το αγόρι with an είναι "is" in it.
Though bear in mind that "this" in English can be a pronoun, standing by itself (e.g. "This is a mountain") or it can be a demonstrative determiner, standing in front of a noun (e.g. "This mountain is dangerous").
The first case is αυτό alone but the second one needs a definite article as well, e.g. αυτός ο άντρας, αυτή η γυναίκα, αυτό το παιδί, αυτό το αγόρι, αυτό το βουνό "this man, this woman, this child, this boy, this mountain".