What must a fasting girl, aged 15-16, do if she drank, presuming the adhan had not been called, then it became clear to her that it had indeed been called?

You drank after dawn, and you are required to make up that day according to the majority of scholars. There is no sin upon you, Allah willing, due to your doubt about the advent of dawn. Some scholars, like Sheikh Al-Uthaymeen, held the view that you are not required to make up the day if you doubted the advent of dawn and then ate, and it later became clear that it had dawned, citing the Almighty’s saying: {And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]}.