Two elements are studied. One has atomic number X and one has atomic number X+2. It is known that element X is a halogen. To what group does the X+2 element belong?

Group 1 (or IA)

Explanation:

If element X is a halogen, then it belongs to the group 17 (or VIIA, under a different notation).

For each extra unit of atomic number, the group number increases by 1. That means that the X+1 element would belong to the group 18 (or VIIIA). The X+2 element would thus belong in the group 1 (or IA) one period higher (higher as in numeric value, not as in position in the periodic table).

i believe it is-

d.) gravity pulled particles of dust and gas together to form planets.

it’s previous models   because when i picked that son of a gun’s answer it was wrong look for previous models because they might make it a different letter