Verified answer

zu = to -- The boy listens (to) or hears.

der Junge = the boy das Junge = the young animal, there is no female form of this noun

"Zu" is the prefix of the verb. The actual verb is zuhoren, but since it is conjugated, the prefix is thrown at the end. Yes, the hint to the noun gender is "der". It tells you that the gender is masculine. If it was feminine, it would be "die" and if it was neutral it would be "das"

"zu" is part of the verb "zuhören" ('to listen') : "ich höre zu", "du hörst zu", "er hört zu", ...

"Junge" is masculine, so it's "der Junge". In contrast to "das Mädchen", which is neutral.