Answer:
The difference in the phase angle between any two waves at any given position along the waves.
Explanation:
Phase difference describes the difference in the phase angle between any two waves at any given position along the waves. The equation of wave is given by :
Wave 1, 
Wave 2, 
Where
A is the amplitude of wave
is the angular frequency
is the phase difference between two waves
If
is the path difference, it is given by :

Hence, the correct option is (a) "the difference in the phase angle between any two waves at any given position along the waves".