**Answer:**

**Streamline and Equipotential Line in a Flow Field**

In fluid dynamics, a streamline and an equipotential line are two important concepts used to understand and analyze the flow of fluids. Let's discuss each of these concepts in detail and understand why they are perpendicular to each other.

**Streamline:**
A streamline is an imaginary line that represents the path followed by a fluid particle in a flow field. It is a line that is tangent to the velocity vector of the fluid at every point along its path. In other words, a streamline is a line that is always parallel to the local velocity vector of the fluid.

**Equipotential Line:**
An equipotential line is an imaginary line that represents the points in a flow field that have the same potential. In fluid dynamics, potential refers to the Bernoulli potential, which is a measure of the total energy per unit mass of the fluid particle. An equipotential line is a line that joins all the points of equal potential in the flow field.

**Perpendicular Relationship:**
Now, let's consider a fluid flow field and imagine a streamline passing through it. Since a streamline is always parallel to the local velocity vector, it means that at every point along the streamline, the velocity vector and the tangent to the streamline are pointing in the same direction.

On the other hand, an equipotential line represents points of equal potential. The gradient of the potential is always perpendicular to the equipotential lines. The gradient of a scalar field is a vector that points in the direction of the maximum rate of change of the scalar field. In the case of the potential field, the gradient vector points in the direction of maximum decrease in potential.

Now, since the streamline is parallel to the velocity vector and the equipotential line is perpendicular to the gradient of the potential, it means that the streamline and equipotential line must be perpendicular to each other. This perpendicular relationship holds true for any point in the flow field.

Therefore, the correct answer to the given question is option B - Streamlines and equipotential lines are perpendicular to each other in a flow field.