Which of the following electron transition in a hydrogen atom will require the largest amount of energy

As the energy level increases, the energy gap decreases progressively. So, the maximum energy for electron transition is required when the electron jumps from n=1 to n=2.

$\mathrm{E}\propto \left[\frac{1}{{\mathrm{n}}_1^2}-\frac{1}{{\mathrm{n}}_2^2}\right]$