Correct Option
The correct option is (a).
Explanation
Nitrogen ($N_2$) is the primary constituent of the Earth's atmosphere and a critical nutrient for living organisms. However, its chemical stability in gaseous form dictates how it interacts with biological systems, particularly plants.
Statement-wise Analysis
- Statement 1 is Correct: Nitrogen is the most plentiful gas in the atmosphere, constituting approximately 78% of the air by volume. It is followed by Oxygen (~21%), Argon (~0.9%), and Carbon Dioxide (~0.04%).
- Statement 2 is Incorrect: Most plants cannot utilize atmospheric nitrogen directly because the nitrogen molecule ($N_2$) is held together by a strong triple bond, making it relatively inert. Plants absorb nitrogen primarily through their roots in the form of soluble compounds like nitrates ($NO_3^-$) and ammonium ($NH_4^+$). These forms are made available through nitrogen fixation carried out by specific bacteria (e.g., Rhizobium in root nodules of legumes) or through physical processes like lightning.
- Statement 3 is Incorrect: Nitrogen is not toxic to plants; rather, it is an essential macronutrient required for their survival. It is a fundamental component of amino acids (the building blocks of proteins), chlorophyll (essential for photosynthesis), and nucleic acids (DNA and RNA). A deficiency in nitrogen leads to stunted growth and yellowing of leaves (chlorosis).
Key Takeaway: Although Nitrogen is the most abundant gas in the atmosphere, it is biologically unavailable to plants in its gaseous form and must be converted (fixed) into nitrates or nitrites to be absorbed and utilized for growth.