What transition in the hydrogen spectrum would have the same wavelength as the Balmer transition n = 4 to n = 2 of He⁺ spectrum?

The decreasing order of electron affinity of F, Cl, Br is F > Cl > Br

The basic nature of the hydroxides of group 13 (Gr. III B) decreases progressively down the group.

Pick out the isoelectronic structures from the following
I. $$CH_3^+$$
II. $$H_3O^+$$
III. $$NH_3$$
IV. $$CH_3^-$$