Answer: Last option.
Step-by-step explanation:
 The equation of a circle in Center-radius form is:
 
 Where the center is at the point (h,k) and "r" is the radius.
 If the center of this circle is at the origin, means that:
 
 Since the radius is 4, then:
 
 Now we need to substitute these values into the equation of the circle.
 
 Simplifying the equation, we get:
 
 This matches with the last option.