Vultures which used to be very common in Indian countryside some years ago are rarely seen nowadays. This is attributed to

The correct answer is B. Vultures declined sharply in the Indian subcontinent in the 1990s because of diclofenac, a veterinary NSAID given to cattle for pain and inflammation. Vultures feeding on carcasses of treated cattle ingested the drug, which caused visceral gout and renal failure, and the population of Gyps species fell by over 90 percent.

Why the others fail:

• A is wrong. No invasive species destroying nesting sites explains the decline. The problem was chemical poisoning, not habitat loss.
• C is wrong. Food was not scarce. Dead cattle were plentiful, which is exactly why the drug reached vultures through their diet.
• D is wrong. Vulture deaths were not from a widespread natural disease. The toxic link to diclofenac was established by researchers, and the cause was human-introduced.

The core concept is bioaccumulation of veterinary drug residues through the food chain. It links to the Environment and Ecology syllabus, and to Mains on the trade-off between livestock healthcare and biodiversity. Note that veterinary diclofenac was banned in 2006 and the species has been slowly recovering since, though the ban is an updated fact relative to 2012.

Trap: aspirants pick D because the question says "fatal disease". Remember that the cause was a drug, not a pathogen.