In which year of Tiberius Caesar's reign did God's word come to John in the wilderness?

Answer: The fifteenth

In the fifteenth year of Tiberius, with Pilate governor of Judea, the word of God came to John son of Zechariah.

“In the fifteenth year of the reign of Tiberius Caesar, while Pontius Pilate was governor of Judea, Herod tetrarch of Galilee, his brother Philip tetrarch of Ituraea and Trachonitis, and Lysanias tetrarch of Abilene, during the high priesthood of Annas and Caiaphas, the word of God came to John son of Zechariah in the wilderness.”— Luke 3:1-2 (BSB)