E-mail accounts administered by an employer belong to them, and they

E-mail accounts administered by an employer belong to them, and they can be seized and used as evidence against the employee.

(A) an employer belong to them, and they can be seized and used - Incorrect
them ideally refers to employer which is in singular form - so wrong

(B) employers belong to them, who can seize and use it - incorrect
Since email accounts is plural - we can't use 'it' to refer to email accounts.

(C) an employer belong to the employer, who can seize and use the accounts - correct

(D) an employer belong to the employer, whom can seize and use the accounts - Incorrect
whom is the objective form of who. But employer is used as subject which performs the action of seize and use the accounts as evidence. So usage of whom to refer to employer is incorrect.

(E) the employers belong to them, who can seize and use it. Same as B.

Answer C