The correct answer is: A. Monomode fiber.
Monomode fiber is a type of optical fiber that has a very small core diameter, typically around 9 microns. This makes it ideal for use in interferometric instruments, which rely on the interference of light waves to measure very small distances.
Multimode fiber, on the other hand, has a larger core diameter, typically around 50 microns. This makes it less suitable for use in interferometric instruments, as the light waves are more likely to spread out and interfere with each other.
Birefringent fiber is a type of optical fiber that has two different refractive indices, one for light polarized along the axis of the fiber and one for light polarized perpendicular to the axis of the fiber. This property can be used to create fiber optic sensors, but it is not typically used in interferometric instruments.
Coated fiber is a type of optical fiber that has a thin layer of material applied to the outside of the fiber. This coating can protect the fiber from damage and can also improve its performance in certain applications. However, it is not typically used in interferometric instruments.
In conclusion, monomode fiber is the best type of fiber to use in interferometric instruments. It has a small core diameter, which makes it ideal for this type of application.