(d) Assertion (A) is false, but Reason (R) is true.
In large animals, oxygen cannot easily reach all body parts by simple diffusion; a transport system with respiratory pigments (like haemoglobin) is needed. The Reason correctly states that respiratory pigments carry oxygen to body tissues.
The Assertion is false because large/complex animals have a problem with oxygen delivery — diffusion alone is insufficient over large distances, which is why they evolved circulatory systems and respiratory pigments. The Reason is true: haemoglobin (a respiratory pigment in RBCs) picks up oxygen in the lungs and carries it to tissues. Since A is false and R is true, option (d) is correct.