The term 'The Evil Quartet' refers to:

The correct answer is Option (4) → Four major causes of biodiversity loss

Species are now being lost much faster than before, mainly because of human activities. The book groups the causes into four, and calls them ‘The Evil Quartet’: habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinctions. So the term means the four major causes of biodiversity loss.

NCERT Extract:
Loss of Biodiversity – “… The accelerated rates of species extinctions that the world is facing now are largely due to human activities. There are four major causes (‘The Evil Quartet’ is the sobriquet used to describe them). (i) Habitat loss and fragmentation: This is the most important cause driving animals and plants to extinction.”