The question requires us to identify the analogy of conductivity in heat transfer in the context of fluid flow. Let's break down the concepts:

1. Conductivity in Heat Transfer: Thermal conductivity is a property of a material that indicates its ability to conduct heat. It is a measure of how easily heat can pass through a material.
2. Fluid Flow Properties: In fluid mechanics, viscosity is a measure of a fluid's resistance to gradual deformation by shear stress or tensile stress. Viscosity is the property that describes how "thick" or "sticky" a fluid is, affecting how it flows.

The key analogy here is to equate the ease of heat transfer (conductivity) with the ease of flow (viscosity) in a fluid. Just as high conductivity allows for easy heat flow, low viscosity allows for easy fluid flow.

Now, let's analyze the given options:

• Viscosity of Fluid: This is directly analogous because it reflects how easily a fluid flows, similar to how conductivity reflects how easily heat transfers.
• Density of Fluid: Density refers to mass per unit volume and does not directly correlate with thermal conductivity.
• Velocity of Fluid: Velocity indicates the speed of flow, affected by viscosity but is not analogous to conductivity.
• Pressure of Fluid: Pressure is the force exerted per unit area and does not relate directly to thermal conductivity.

Based on the explanation above, the correct choice that represents the analogy of conductivity in heat transfer to fluid flow is Viscosity of fluid.

Therefore, the answer to the question is Viscosity of fluid.