Which one among the following is not an example of homologous organs ?

Homologous organs are those that share a common anatomical origin and basic structural plan but perform different functions, indicating divergent evolution. In contrast, analogous organs have different origins but perform similar functions due to convergent evolution.

• Option B is correct: Wings of birds (vertebrate forelimbs with bones) and wings of butterflies (chitinous extensions of the integument) have entirely different structures and origins despite both being used for flight. Therefore, they are analogous, not homologous.
• Option A: Both thorns of Bougainvillea and tendrils of Cucurbita are modifications of the axillary bud, making them homologous.
• Options C and D: Forelimbs of horses, humans, bats, and dolphins all share the same basic pentadactyl skeletal structure (humerus, radius, ulna, carpals, etc.), adapted for different purposes like running, grasping, flying, or swimming. Thus, they are homologous.