ANS:B - stream line

he imaginary line drawn such that the tangents at its all points indicate the direction of the velocity of the fluid particles at each point is called a "stream line." Streamlines are a fundamental concept in fluid dynamics. They represent the instantaneous velocity vector of fluid flow at every point along the line. In other words, at any given point on a streamline, the tangent to the streamline indicates the direction of the fluid velocity at that point. The other terms mentioned also relate to fluid flow:

• Path line: The actual path traced by a fluid particle as it moves through the flow field over time.
• Potential line: This term is less common and typically refers to lines in a flow field where the velocity potential is constant. It's related to potential flow theory, where the flow is assumed to be irrotational and the velocity field can be derived from a scalar potential function.
• Streak line: The streak line is the locus of points occupied at earlier times by fluid particles that pass through a particular point in the flow at an instant. In other words, it represents the history of fluid particles that have passed through a specific point in the flow field.