The correct answer is option 2, Venturi tube.

1. A barometer is a device used to measure atmospheric pressure and is not suitable for measuring flow velocities in a fluid.

2. The Venturi tube is a device commonly used to measure flow velocities in an incompressible fluid. It works based on the principle of fluid flow through a constriction. As the fluid passes through the narrow section of the tube, its velocity increases, and this change in velocity can be used to determine the flow rate.

3. A strain gauge is a sensor used to measure strain or deformation in an object. It is not specifically designed for measuring flow velocities in a fluid.

4. A manometer is a device used to measure pressure differences between two points in a fluid. While it can indirectly provide information about flow velocities, it is not as accurate or direct as a Venturi tube.

In conclusion, the Venturi tube is the most suitable option for measuring flow velocities in an incompressible fluid.