Definition of a streamline
A streamline is a concept used in fluid dynamics to describe the path followed by a small imaginary particle as it moves through a fluid. It is a line that is drawn tangent to the velocity vector at every point along its path.

Understanding the options
Let's analyze each option to understand why option D is the correct answer:

a) Which is along the path of the particle:
This option is partially correct. A streamline does represent the path followed by a particle, but it is not just any path. It is a path that is tangent to the velocity vector at every point along the line. So, this option is not completely accurate.

b) Which is always parallel to the main direction of flow:
This option is incorrect. While a streamline may be parallel to the main direction of flow at some points, it is not always the case. The streamline can change direction and deviate from the main flow direction depending on the characteristics of the flow.

c) Along which there is no flow:
This option is incorrect. A streamline represents the path of a moving particle, so there is flow along the streamline. In fact, the velocity vector at any point on the streamline gives the speed and direction of flow at that point.

The correct answer: Option D
d) On which tangent drawn at any point gives the direction of flow:
This option is the correct answer. When a tangent is drawn at any point along a streamline, it represents the direction of flow at that particular point. The velocity vector, which is tangent to the streamline, gives the speed and direction of flow at that point. Therefore, this option accurately describes the concept of a streamline.

Conclusion
A streamline is a line that represents the path followed by a small imaginary particle as it moves through a fluid. It is a line along which a tangent drawn at any point gives the direction of flow at that point. The velocity vector, which is tangent to the streamline, provides information about the speed and direction of flow at each point along the line.