The correct answer is: Silica.
Index-guided photonic crystal fiber (PCF) is a type of optical fiber that uses a periodic variation in the refractive index of the core and cladding to confine light. The dark areas in the PCF structure are air holes, which provide a low-index region for light to propagate through. The white areas are silica, which has a higher refractive index than air. This difference in refractive index between the air holes and the silica cladding causes light to be guided along the axis of the fiber.
Option A is incorrect because air is the material that fills the air holes in the PCF structure.
Option B is incorrect because silica is the material that makes up the cladding of the PCF structure.
Option C is incorrect because water is not a material that is used in PCF structures.
Option D is incorrect because plasma is not a material that is used in PCF structures.