What is the minimum concentration of H2S which can cause death if a person is exposed for even an instant?

Correct Answer: Option D — 1000 ppm

At 1000 ppm, Hydrogen Sulfide acts as a chemical asphyxiant that causes immediate paralysis of the respiratory center in the brain. This concentration leads to instantaneous collapse and death after a single breath. There is no time for escape; immediate, fatal respiratory arrest occurs upon exposure to this extreme level.