• Assertion (A) is True: In natural (free) convection, temperature cannot be treated as a passive scalar (passive admixture) because local temperature variations directly alter fluid density ($\Delta \rho = -\rho \beta \Delta T$).
• Reason (R) is True: Spatial density inhomogeneities in a gravitational field generate buoyancy forces (buoyant acceleration $g \beta \Delta T$), actively coupling the temperature field to the Navier-Stokes momentum equations.
• Relationship: Because temperature-induced density gradients generate buoyancy forces that drive fluid motion, Reason (R) directly provides the physical cause for 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)