Who announced to Zechariah that Elizabeth would bear a son?

Answer: Gabriel

The angel told Zechariah his wife Elizabeth would bear a son named John, saying, 'I am Gabriel. I stand in the presence of God.'

“But the angel said to him, “Do not be afraid, Zechariah, because your prayer has been heard. Your wife Elizabeth will bear you a son, and you are to give him the name John. “I am Gabriel,” replied the angel. “I stand in the presence of God, and I have been sent to speak to you and to bring you this good news.”— Luke 1:13, 1:19 (BSB)