Which of the following pairs is not an example of homology?

The correct answer is Option (2) → Wings of butterfly and wings of bird

Homologous organs have the same basic structure and origin but may do different jobs (divergent evolution). Analogous organs do the same job but are built differently (convergent evolution).

Option (1) Forelimbs of whale and forelimbs of cheetah — An example of homology. Same bones (humerus, radius, ulna, carpals) used for swimming and running.

Option (2) Wings of butterfly and wings of bird — Not an example of homology. Both are for flying, but the insect wing and the bird's forelimb have different structures: they are analogous.

Option (3) Forelimbs of human and forelimbs of whale — An example of homology. Human and whale forelimbs both have humerus, radius, ulna, carpals, metacarpals and phalanges. The human arm handles objects and the whale flipper swims, so the same structure does different work. This is divergent evolution.

Option (4) Thorn of Bougainvillea and tendril of Cucurbita — An example of homology. Both are modified stems.

NCERT Extract:
What are the Evidences for Evolution? – “… Homology indicates common ancestry. Other examples are vertebrate hearts or brains. In plants also, the thorn and tendrils of Bougainvillea and Cucurbita represent homology (Figure 6.3a). Homology is based on divergent evolution whereas analogy refers to a situation exactly opposite. Wings of butterfly and of birds look alike. They are not anatomically similar structures though they perform similar functions. Hence, analogous structures are a result of convergent evolution - different structures evolving for the same function and hence having similarity.”

Figure 6.3 Example of homologous organs in (a) Plants and (b) Animals