Question
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Which has the lowest electron affinity among the following?
[A] I
[B] Br
[C] Cl
[D] F

Answer
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Hint:
The least electronegative element in the option will be the correct answer for this question. Down the group, electronegativity decreases. As we know, down the group atomic size increases, the effective nuclear charge decreases which means the outer orbital electrons are shielded by the inner electrons hence does not attract an electron readily.

Complete step by step answer:
We know that electron affinity is the energy released by a neutral atom or molecule when an electron is attached to it and results in the formation of a negative ion.
In simpler words, we can say that electron affinity is the affinity of an atom to gain an electron and is denoted as ${{E}_{ea}}$.
We know that when an electron is added to an atom, the atom gains a negative charge and this causes a release in energy but when releasing an electron from the atom, it requires energy.
We can write this in form of a reaction as-
     \[X(g)+{{e}^{-}}\to {{X}^{-}}(g)+energy\]
Here ‘X’ is an arbitrary atom.
Electron affinity varies greatly across the period table but there are a few generalised trends.
Generally non-metals tend to have a higher positive electron affinity than metals and atoms which are more stable in their anion forms than the neutral form have a higher electron affinity.
General trend of electron affinity is that it increases across the period till group 17. As we know, that group 17 elements have 7 electrons in their valence shell. If they attract an electron, they will release a higher amount of energy (as they attain noble gas configuration, a fulfilled valence shell) compared to the group 1 elements.
However, for the group 18 elements, as we know that they already have fulfilled valence shells, they tend to reject accepting or losing an electron.
There is no regular trend for electron affinity while moving downward along the group but among the halogens, Chlorine has the highest electron affinity followed by fluorine, bromine and iodine.
As we know, fluorine is a small atom, if it accepts an electron there will be electron-electron repulsion in the atom making it unstable however, for chlorine the outermost orbital is 3p which makes it easier to accept an electron due the availability of space. Whereas for bromine and iodine, due to the increase in their sizes, the electrons face a decrease in effective nuclear charge therefore, electrons are not attracted by these atoms.

The correct answer is option [A].

Note:
Electron affinity basically means the willingness of the atom to accept an electron. An atom will be willing to gain an electron only if it makes it stable. Therefore, there are certain elements for which the first electron affinity is negative but the second electron affinity is positive like oxygen.