Elements considered

The first four elements of group 17 are: $\mathrm{F,\ Cl,\ Br,\ I}$

We check which properties show irregularity in their trend.

(A) Covalent radius

Down the group, atomic size increases because a new shell is added each time.

Hence covalent radius follows: $\mathrm{F < Cl < Br < I}$

This is a regular increase. So no irregularity.

(B) Electron affinity

In general, electron affinity becomes less negative down a group, but fluorine is an exception.

Expected trend would suggest fluorine should have the most negative electron affinity, but actually: $\mathrm{Cl > F}$ in magnitude of electron affinity.

This happens because the added electron in fluorine enters the very compact $2p$ orbital, where electron-electron repulsion is high.

So electron affinity shows an irregularity.

(C) Ionic radius

For halide ions: $\mathrm{F^- < Cl^- < Br^- < I^-}$

Ionic radius increases regularly down the group due to increase in number of shells.

So no irregularity.

(D) First ionization energy

Down the group, first ionization energy decreases regularly because atomic size increases and shielding increases.

Thus: $\mathrm{F > Cl > Br > I}$

This is a regular trend, not irregular.

Conclusion

Only electron affinity shows irregularity among the first four elements of group 17.

Therefore the correct choice is: $\boxed{\text{B only}}$

Match with given options

Option C corresponds to B only.

Comparison with stored answer

Stored correct answer = C

Our derived answer = C

So they agree.