:

read the given expression.

x = atomic number − number of core electrons

which of the following explains the identity of x and its trends down a group?
x is the effective nuclear charge, and it remains constant down a group.
x is the screening constant, and it remains constant down a group.
x is the effective nuclear charge, and it increases down a group.
x is the screening constant, and it increases down a group.

Answer;

X is the effective nuclear charge, and it remains constant down a group.

Explanation;Effective nuclear charge is the actual nuclear charge (atomic number) minus the number of inner-core electrons which "shield" the valence electrons from feeling the full attraction.Effective nuclear charge increases left to right across a period, because atomic number is increasing while inner-core electrons are constant .The effective nuclear charge remains constant down a group, because atomic number increases, but so do the number of core electrons