• Statement (I) is correct: In upland arid and semi-arid calcareous soils with high pH ($pH \gt 7.5$) and high free calcium carbonate ($CaCO_3$), iron precipitates into insoluble ferric hydroxides and carbonates ($Fe(OH)_3$), severely suppressing bioavailable divalent ferrous ($Fe^{2+}$) and triggering widespread lime-induced iron chlorosis in upland crops like groundnut and chickpea.
• Statement (II) is correct: Molybdenum (Mo) is a crucial metallic cofactor present in nitrogenase (within the FeMo-cofactor catalyzing biological atmospheric $N_2$ reduction) and nitrate reductase (catalyzing the reduction of nitrate $NO_3^-$ to nitrite $NO_2^-$), making it indispensable for leguminous symbiotic nitrogen fixation and microbial nitrate assimilation.
• Conclusion: Both Statement (I) and Statement (II) are correct. (Correct answer: Option 1)

• Assertion (A) is true: Soil dehydrogenase enzyme activity exists strictly within intact viable microbial cells and serves as an established index of total oxidative microbiological activity and biological respiration in soils.
• Reason (R) is true & correct explanation: Dehydrogenases catalyze the oxidation of soil organic matter by transferring hydrogen protons ($H^+$) and electrons from organic electron donors to specific intracellular electron acceptors (NAD/NADP, cytochromes, and tetrazolium salts in assays). This electron transport mechanism directly accounts for why dehydrogenase activity reflects oxidative metabolism.
• Conclusion: Both (A) and (R) are true and (R) is the correct explanation of (A). (Correct answer: Option 1)

• Mathematical Conversion of Percent to Parts Per Million (PPM):

$$1\% = \frac{1}{100} = \frac{10,000}{1,000,000} = 10,000\text{ ppm}$$

• Calculation for 2% Solution:

$$2\% = 2 \times 10,000\text{ ppm} = 20,000\text{ ppm}$$

• Conclusion: A 2% concentration solution is equivalent to 20,000 ppm. (Correct answer: Option 3)

• Invention of the pH Scale:
• The concept of pH was introduced in 1909 by Danish biochemist S.P.L. Sørensen (Søren Peter Lauritz Sørensen) while working at the Carlsberg Laboratory in Copenhagen.
• He defined pH as the negative logarithm of hydrogen ion activity:

$$\text{pH} = -\log_{10} [\text{H}^+]$$

where 'p' stands for the German Potenz or French puissance (meaning power or potential) and 'H' represents the hydrogen ion concentration.

• Conclusion: SPL Sørensen invented the pH scale (Option 1).

Correct option: (1) SPL Sorenson