What type of instrument is primarily used for measuring fluid pressure?

A manometer is primarily used for measuring fluid pressure, making it the correct answer. This instrument operates by balancing a column of liquid against the pressure of the fluid being measured, providing a direct and visual representation of pressure differences. Manometers can be found in various forms, such as U-tube manometers or digital manometers, and are widely used in various applications to measure gas, liquid, or vapor pressure.

In contrast, a caliper is designed for measuring lengths or dimensions rather than fluid pressure, making it unsuitable for this application. A multimeter is an instrument used for measuring electrical parameters such as voltage, current, and resistance, thus not appropriate for pressure measurements. An oscilloscope is used to visualize electrical signals and waveforms, so it also does not meet the requirement for measuring fluid pressure. Therefore, the manometer stands out as the appropriate instrument for this specific purpose.