• Assertion (A) is True: The potassium activity ratio ($AR^K = \frac{a_{\text{K}}}{\sqrt{a_{\text{Ca}} + a_{\text{Mg}}}}$) is widely used in soil chemistry instead of absolute K concentration to represent the immediate thermodynamic intensity factor ($I$).
• Reason (R) is True: Plant root membrane transport of potassium is competitively governed by the simultaneous chemical activities of divalent cations ($ ext{Ca}^{2+}$ and $ ext{Mg}^{2+}$) in the soil solution.
• Relationship: Because root potassium absorption depends competitively on calcium and magnesium activity, $AR^K$ provides the physiologically relevant intensity metric, directly explaining Assertion (A).
• Conclusion: Both (A) and (R) are true 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)

• Microbial Temperature & Trophic Classifications:
• Mesophiles: Microorganisms with optimum growth temperatures between $20^\circ\text{C}$ and $35 - 40^\circ\text{C}$ (comprising the majority of soil bacteria and actinomycetes).
• Psychrophiles: Cold-tolerant organisms capable of growth below $10^\circ\text{C}$ (optimum $\le 15^\circ\text{C}$).
• Chemoautotrophs: Derive metabolic energy by oxidizing simple inorganic compounds ($NH_4^+, NO_2^-, Fe^{2+}, S$).
• Heterotrophs: Obtain both energy and cellular carbon by oxidizing complex organic compounds.
• Conclusion: Statements (A), (B), (C), and (D) are all correct. (Correct answer: Option 3)