ANS:A - normal

An equipotential line in fluid dynamics is normal to the velocity vector at every point.

Explanation:

1. Equipotential Line:
   • An equipotential line in fluid flow represents a line where the potential (such as pressure or velocity potential) remains constant.
   • In the context of fluid dynamics, it is often associated with the pressure potential, where the pressure is constant along the equipotential line.
2. Relationship with Velocity Vector:
   • The velocity vector in fluid flow is tangent to the streamlines. Streamlines are imaginary lines that are everywhere tangent to the velocity vector at a given instant.
3. Normality to Velocity Vector:
   • An equipotential line is perpendicular (normal) to the velocity vector at every point where they intersect.
   • This relationship arises because in fluid flow, pressure (and thus potential) gradients drive fluid motion along streamlines, which are tangent to the velocity vector.
4. Options Analysis:
   • Normal: Correct term in fluid dynamics indicating perpendicularity.
   • Abnormal: Not a standard term in this context.
   • Tangential: Describes the direction of streamlines in relation to the velocity vector, not equipotential lines.

Conclusion: