The correct answer is Sulphur - a component of nucleic acids. Explanation & Key Takeaways: • Plants require a number of inorganic nutrients. Some of these are macronutrients, which the plants need in relatively large amounts, and others are micronutrients, which are required in trace amounts. • Nitrogen • It is present in various coenzymes, hormones, ATP, etc. • It is a vital constituent of vitamins, nucleic acids, proteins, and many others. Hence, pair 1 is correctly matched. • Deficiency of nitrogen leads to the complete suppression of flowering and fruiting, impaired growth, and development of anthocyanin pigmentation in stems. • Magnesium • It is absorbed by plants in the form of divalent Mg2+. • It activates the enzymes of respiration, photosynthesis. • It is involved in the synthesis of DNA and RNA. • It is a constituent of the ring structure of chlorophyll and helps to maintain the ribosome structure. Hence, pair 2 is correctly matched. • Phosphorous • It boosts fruit ripening and root growth in a healthy manner by helping translocation of carbohydrates. • They are found abundantly in fruits and seeds. • It is a constituent of cell membranes, certain proteins, all nucleic acids, and nucleotides, and is required for all phosphorylation reactions. Hence, pair 3 is correctly matched. • Deficiency of Phosphorus leads to premature fall of leaves and they turn purplish or dark green in color. • It is part of the RNA and DNA structures, which are the major components of genetic information. • Sulphur • Plants obtain sulphur in the form of sulphate. • It is present in two amino acids – cysteine and methionine. • It is the main constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A, and ferredoxin). Hence option 4 is incorrectly paired.