What type of human activity is endangering the continued existence of…

The correct answer is Option (1) → Over-exploitation

The book lists the main causes of the loss of biodiversity: habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinctions. Over-exploitation means taking more from nature than it can replace, when ‘need’ turns to ‘greed’. The example given for marine fish is that many of their populations are over harvested.

Option (1) Over-exploitation — Correct. Many marine fish populations are over harvested around the world. Catching more fish than can be replaced puts some commercially important species at risk, and this is over-exploitation.

Option (2) Pollution — Incorrect. Pollution degrades habitats and threatens many species. But the danger the book names for the commercially important marine fishes is over-harvesting, not pollution.

Option (3) Habitat loss and fragmentation — Incorrect. Habitat loss and fragmentation is the clearing or breaking up of living space, as when tropical rain forests are cut down. The threat to these fishes comes from catching too many of them, not from losing their habitat.

Option (4) Global warming — Incorrect. Global warming changes the climate over large areas. It is not the cause that the book gives for the decline of these marine fishes, which is over-harvesting.

NCERT Extract:
Loss of Biodiversity – “… (ii) Over-exploitation: Humans have always depended on nature for food and shelter, but when ‘need’ turns to ‘greed’, it leads to over-exploitation of natural resources. Many species extinctions in the last 500 years (Steller’s sea cow, passenger pigeon) were due to overexploitation by humans. Presently many marine fish populations around the world are over harvested, endangering the continued existence of some commercially important species.”