The increasing order of electron affinity is :

$\mathrm{Al}<\mathrm{N}<\mathrm{O}<\mathrm{Cl}$ 

Let's organise the elements in accordance with how the periodic table has them listed.
Lithium Carbon Boron Beryllium Nitrogen Oxygen Fluorine
chlorinated aluminum
The electron affinity increases as we move from lithium to fluorine, and it decreases as we move from fluorine to chlorine. Therefore, the electron affinity of chlorine should be greater than that of oxygen, greater than that of nitrogen, and greater than that of aluminum.
The size grows as we descend, and as a result, the tendency to gain electrons, or electron affinity, declines. Nonmetals gain electrons; metals do not. Aluminum is thus at the final position.