Which element is most reactive among aluminium, boron, calcium and beryllium? Give reason.

Well, if you check the reactivity series, calcium is more reactive than magnesium. This means there's a a trend of increasing reactivity as we go down the group. As beryllium is above magnesium, it must be the least reactive. Then magnesium is in the middle and the most reactive has to be calcium.

Aluminium because highl elecronegativety element and more metalic hign melting and boling point from the other

if you're asking what comes out, it's steam.