We know from Rydberg's equation the wavelength λ of emission spectrum when electrons move from n₂ energy level to n₁ energy level, is given by,

For option (a), we have

For option (b), we have

For option (c ), we have

For option (d), we have

Thus, in option (d) 1/λ has the maximum value and thus λ is minimum.

Hence, option (d) is the correct answer.