In which of the following pairs, the first element is least reactive and second has least atomic number?

In the pair of Br and F, the first element is the least reactive and the second has the least atomic number. 
Halogens are present in group-17 of the periodic table. F, Cl, Br, I, F has the lowest atomic number (Z = 9 ). 

The reactivity of halogens depends on electronegativity. Electronegativity is the property that describes the tendency of an element to attract the shared pair of electrons in a bond. 

As the number of shells increases, the tendency to attract electrons decreases because the distance of the nucleus from the outermost electron increases. 

When we move from top to bottom in halogen groups, electronegativity from ‘F’ to ‘Br’ decreases. Thus, ‘Br’ is the least reactive.