The correct option is
D Retina
Muller's fibres are a kind of glial cells found in retina. They maintain the structure of retina and provide functional stability. They regulate extracellular environment by controlling the uptake of neurotransmitters, regulation of potassium levels, glycogen storage etc. These fibres form a continuous and dense meshwork. They are found in the retina of vertebrates.
So, the correct answer is 'Retina'.