Correct Answer - Option 4 :

Stream line
__Concept:__

Streamlines are the lines drawn through the flow field in such a manner that the velocity vector of the field at each and every point on the streamline is tangent to the streamline at that instant.

So, the curve that is everywhere tangent to the instantaneous local velocity vector is ‘streamline’

The equation of streamline is given by

\(\frac{{dx}}{u} = \frac{{dy}}{v} = \frac{{dz}}{w}\)