Question

the lightest transition metal -->
the heaviest alkali metal -->
the third halogen element -->
the fourth noble gas element -->
kr sc fr br

Explanation:

The lightest transition metal is Scandium (Sc). The heaviest alkali - metal is Francium (Fr). The third halogen element is Bromine (Br). The fourth noble - gas element is Krypton (Kr).

Answer:

The lightest Transition Metal --> Sc
The heaviest Alkali Metal --> Fr
The third Halogen element --> Br
The fourth Noble Gas element --> Kr