Who asked Herod for John the Baptist's head on a platter?

Answer: Herodias' daughter

Prompted by her mother Herodias, the daughter who had danced for Herod asked for John's head on a platter.

“On Herod’s birthday, however, the daughter of Herodias danced before them and pleased Herod so much that he promised with an oath to give to her whatever she asked. Prompted by her mother, she said, “Give me here on a platter the head of John the Baptist.””— Matthew 14:6-8 (BSB)