The correct answer is option 4: Flint glass. Flint glass is a type of glass that is commonly used for making optical instruments.

To understand why flint glass is used for optical instruments, let`s examine the other options.

Option 1: Pyrex glass is known for its ability to withstand high temperatures and thermal shocks, making it ideal for use in laboratory glassware and cookware. However, it is not commonly used in optical instruments.

Option 2: Soft glass generally refers to glass with a low softening point, which makes it easier to manipulate and shape. While soft glass may be used for certain optical applications, it is not as commonly used as flint glass.

Option 3: Hard glass usually refers to glass that has been treated to be more durable, often through a process called annealing. While hard glass may be used for some optical components, it is not as commonly used as flint glass in optical instruments.

Option 4: Flint glass is specifically designed to have certain optical properties, such as a high refractive index. This makes it particularly useful for lenses and prisms used in optical instruments, as it allows for the manipulation and control of light.