$(i)$ Why is Newton's law of gravitation called the universal law?
$(ii)$ What is the significance of this law in nature?

(N/A) $(i)$ Newton's law of gravitation is applicable to all objects in the universe,regardless of their size,mass,or distance,hence it is called the universal law of gravitation.
$(ii)$ The universal law of gravitation is significant because it successfully explains the following phenomena:
$(a)$ The gravitational force that binds us to the Earth.
$(b)$ The motion of the Moon around the Earth.
$(c)$ The motion of planets around the Sun.
$(d)$ The occurrence of tides in the oceans due to the gravitational pull of the Moon and the Sun.