Which is an example of homology?

The correct answer is Option (4) → Thorns of Bougainvillea and tendril of Cucurbita

Option (4) Thorns of Bougainvillea and tendril of Cucurbita — Correct. Thorns of Bougainvillea and tendrils of Cucurbita have the same origin but do different jobs (protection and climbing), so they are homologous; NCERT gives them as its plant example of homology. The other three pairs are analogous.

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.”