The Biosphere is a narrow zone where we find land,water and air together,so in the line why Biosphere is said a narrow zone??

Yes, biosphere is a narrow zone where the lithosphere, hydrosphere and atmosphere meet and interact. In this zone all living organisms including humans are closely attached to each other and to the atmosphere for survival. Biosphere is sandwiched between the relatively thick lithosphere and the troposphere. It is relatively narrow zone.