Which of the following elements is very reactive and needs to lose one electron to have a full outermost shell?

Scandium

Cesium

Iodine

Xenon

Xenon has no reactivity so you know it can't be that one. Lodine has very low reactivity so it also can't be that one. Both Cesium and Scandine have high reactivity so it would have to be one of these two. And since Cesium is in the alkalai group you know it has to be that one.

Cesium because it is the only one that belongs to the alkali group.