• 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)

• Assertion (A) is True: The application of gypsum ($ ext{CaSO}_4\cdot 2 ext{H}_2 ext{O}$) is the standard chemical reclamation practice for sodic (alkali) soils.
• Reason (R) is True: Soluble $ ext{Ca}^{2+}$ provided by gypsum replaces exchangeable $ ext{Na}^+$ on soil clay colloids via cation exchange ($ ext{Soil-Na}_2 + ext{CaSO}_4 \rightarrow ext{Soil-Ca} + ext{Na}_2 ext{SO}_4$), allowing sodium to be leached out as soluble sodium sulfate.
• Relationship: Because $ ext{Ca}^{2+}$ replaces exchangeable sodium to flocculate colloids and eliminate sodicity, Reason (R) directly explains Assertion (A).
• Conclusion: Both (A) and (R) are correct and (R) is the correct explanation of (A) (Option 1).

Correct option: (1) Both (A) and (R) are true and (R) is the correct explanation of (A)

• Standard Oxidizing Agents in Soil & Analytical Chemistry:
• Potassium permanganate ($KMnO_4$): Strong oxidizer used in alkaline permanganate nitrogen estimation (Subbiah & Asija method).
• Potassium dichromate ($K_2Cr_2O_7$): Primary standard oxidizer used in Walkley-Black organic carbon determination.
• Potassium iodate ($KIO_3$): Stable primary standard oxidizing agent in iodometry.
• Potassium bromate ($KBrO_3$): Powerful oxidizing titrant in bromatometry.
• Other reagents: Ferrous chloride and stannous chloride are reducing agents; sodium thiosulphate is a standard reducing titrant. (Correct answer: Option 4)