We know that Mulliken scale considers the electronegativity of an element as the average of the ionization potential and the electron affinity of that element. 

$\mathrm{EN}=\frac{\left(\mathrm{IP}\right)+\left(\mathrm{EA}\right)}{2}$

Now, According to question

$\mathrm{EA}=2\mathrm{EN}-\mathrm{IP}$

$\mathrm{EA}=2{\mathrm{E}}_1-{\mathrm{E}}_2$

Hence, option(a) is the answer.