Look at Gabriel’s wording:
He will go before him in the spirit and power of Elijah to turn the hearts of fathers toward children and the disobedient to the understanding of the righteous, to prepare a people fit for the Lord.”
A) There are many verses in the Gospels that show that John was in the “spirit of Elijah”.
And then look at the Canticle of Zechariah:
And you, child, will be called prophet of the Most High, for you will go before the Lord
* to prepare his ways, to give his people knowledge of salvation through the forgiveness of their sins,
B) John was certainly prophecy of the Coming of the Most High Jesus.
C) John the Baptists role was to “Prepare the Way of the Lord”.
D) John certainly preached a message of repentance for the forgiveness of sins.
I’d say its pretty clear that Zechariah’s and Elizabeth’s son is none other than John the Baptist.