(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
Plasma transports CO₂ in dissolved form because CO₂ is more soluble in water than O₂, so O₂ must be carried by haemoglobin in RBCs.
The textbook states: "Plasma transports food, carbon dioxide and nitrogenous wastes in dissolved form. Oxygen is carried by the red blood corpuscles." The reason CO₂ can dissolve in plasma (water-based) while O₂ cannot is directly explained by the greater solubility of CO₂ in water — making R the correct explanation of A. Choose option (A).