Which of the following element is not a halogen?

The elements of Group VIIA (new Group 17 - F, Cl, Br, I, and At) are called halogens. 'Cs' is present in Group I A (first group) of the modern periodic table. so, it is not halogen.

Halogens are mentioned in the yellow colour in the 17th column of the modern periodic table and 'Cs', an alkali metal, is mentioned in the first group in the red colour. (refer to the image)

Hence, Cs is not ahalogen