When comparing the two elements k and cs , the element with the more negative electron affinity is?

The answer to this question would be: cs 

Electron affinity is determined by the amount of energy released/spent for the atom to add an electron for neutral atoms in gas state. 
Lower periodic table means a bigger radius atom. In this case, the distance between the electron to the nucleus is further too, so the affinity is expected to become lower.

In this question, the K and Cs are located in the first column of the periodic table, but Cs located lower. So, the Cs that located lower in periodic table should have a lower affinity.