The correct answer is option 'D', which states that the instruments used for the measurement of pressure include bellows, diaphragms, and fiber optic pressure sensors. Let's understand each of these instruments in detail:

1. Bellows:
- Bellows are flexible containers or chambers that expand or contract when pressure is applied.
- They are typically made of metal and are designed to withstand high-pressure environments.
- When pressure is applied to a bellows, it expands or contracts, which can be measured and used to determine the pressure.
- Bellows are commonly used in pressure gauges, pressure switches, and pressure transmitters.

2. Diaphragms:
- Diaphragms are thin, flexible membranes that deform when pressure is applied.
- They are often made of materials such as metal, rubber, or silicone.
- Diaphragm pressure sensors use the deformation of the diaphragm to measure pressure.
- When pressure is applied to one side of the diaphragm, it flexes, causing a change in electrical or mechanical output that can be measured.
- Diaphragm pressure sensors are widely used in various applications, including automotive, medical, and industrial sectors.

3. Fiber Optic Pressure Sensors:
- Fiber optic pressure sensors use the principle of light transmission through optical fibers to measure pressure.
- These sensors consist of a diaphragm or a pressure-sensitive element that changes the intensity or wavelength of light passing through the fiber.
- The change in light properties is then detected and converted into an electrical signal for pressure measurement.
- Fiber optic pressure sensors offer advantages such as high accuracy, stability, and resistance to electromagnetic interference.
- They are commonly used in applications where electrical interference or hazardous environments are a concern, such as oil and gas, aerospace, and medical industries.

In conclusion, the correct answer option 'D' is accurate as all three instruments - bellows, diaphragms, and fiber optic pressure sensors - are commonly used for the measurement of pressure. Each instrument employs different principles and technologies to convert pressure into measurable signals, catering to various applications and environments.