The correct option is (a).
Explanation
Plant metabolism involves the synthesis of various organic compounds. While photosynthesis produces carbohydrates using carbon dioxide and water, the synthesis of proteins requires additional essential elements, primarily nitrogen, which must be sourced through specific mechanisms.
Statement 1 is Correct:
Proteins are polymers of amino acids. The fundamental structure of an amino acid includes an amino group ($-\text{NH}_2$), a carboxyl group ($-\text{COOH}$), and a side chain. The amino group contains nitrogen. Therefore, nitrogen is an essential constituent of all proteins.
Statement 2 is Incorrect:
Although the atmosphere consists of approximately 78% nitrogen gas ($\text{N}_2$), plants cannot absorb this gaseous nitrogen directly through their leaves (stomata). The triple bond in molecular nitrogen makes it relatively inert. Plants primarily absorb nitrogen from the soil through their roots in the form of dissolved nitrates ($\text{NO}_3^-$) and nitrites ($\text{NO}_2^-$), or as ammonium ions ($\text{NH}_4^+$).
Statement 3 is Incorrect:
Carbon dioxide and water are the raw materials used in photosynthesis to synthesize carbohydrates (such as glucose), which consist of carbon, hydrogen, and oxygen. However, to synthesize proteins, plants require nitrogen (and often sulphur) in addition to carbon, hydrogen, and oxygen. Since carbon dioxide and water do not provide nitrogen, plants cannot synthesize proteins using these two inputs alone.
Key Takeaway:
Nitrogen is a critical component of proteins, distinct from carbohydrates which are formed mainly of carbon, hydrogen, and oxygen. Plants lack the ability to fix atmospheric nitrogen directly; they rely on soil-based absorption of nitrates and nitrites.