You have been tasked with modifying the structure of an existing employee database to add a new column named 'PhoneNumber' that will store the employees' contact numbers. Which SQL command should you use to accomplish this task?
UPDATE Employees SET PhoneNumber VARCHAR(10);
ALTER TABLE Employees ADD COLUMN PhoneNumber VARCHAR(10);
CREATE TABLE Employees ADD PhoneNumber VARCHAR(10);
MODIFY TABLE Employees ADD COLUMN PhoneNumber VARCHAR(10);