In the ‘rivet popper hypothesis’, the ‘rivet’ signifies:

The correct answer is Option (4) → Species

The rivet popper hypothesis is an analogy given by the ecologist Paul Ehrlich. It explains why the loss of species matters. An airplane stands for the ecosystem, and the thousands of rivets that hold it together stand for the species. Rivets on the wings stand for key species that drive major ecosystem functions.

NCERT Extract:
The Importance of Species Diversity to the Ecosystem – “… In an airplane (ecosystem) all parts are joined together using thousands of rivets (species). If every passenger travelling in it starts popping a rivet to take home (causing a species to become extinct), it may not affect flight safety (proper functioning of the ecosystem) initially, but as more and more rivets are removed, the plane becomes dangerously weak over a period of time.”