Option (b) Nitrogen is the correct answer. The provided context discusses the composition of the atmosphere, mentioning gases like Carbon dioxide, Ozone, and Water vapour. It notes that water vapour can account for up to 4% of the air by volume in the tropics. However, the context does not explicitly state the percentage of Nitrogen. Referring to the NCERT textbook, Nitrogen is the most abundant gas in the atmosphere, constituting approximately 78% of the total volume, while Oxygen constitutes about 21%. Therefore, Nitrogen constitutes the major portion of the atmosphere.