Option (b) 120 km is correct. The textbook explicitly states that 'The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km.' This is a direct factual statement from the chapter 'Composition of the Atmosphere'. Options (a) 90 km, (c) 100 km, and (d) 150 km are incorrect as they do not match the height specified in the NCERT textbook. Additionally, the context mentions that carbon dioxide and water vapour are found only up to 90 km, which is different from the height where oxygen becomes negligible.