When an excited hydrogen atom returned to its ground state, some visible quanta were observed along with other quanta. Which of the following transitions must have occurred?

There must have been a shift from $2\to 1$.
This is due to the fact that when an electron de-excites by moving from a higher level to a lower level, visible quanta are recorded in the Balmer series. The transition $2\to 1$ is produced when the electron de-excites to the ground state after reaching the second level. Now, the quanta released will be in the UV zone.

Hence, the correct option is A.