Ancient History question: Which one factor is often considered a possible reason for the decline of the Harappan Civilization?

The original question-specific evidence states: Environmental changes, especially shifts in river courses, are often suggested as one reason for decline. Changes in river systems is the correct answer to “Which one factor is often considered a possible reason for the decline of the Harappan Civilization”. This evidence connects the required Ancient History fact with its archaeological, geographical, cultural or chronological setting. Harappan evidence must be interpreted from excavated settlements, artefacts, environmental data and comparisons among sites; a single object should not automatically support a wider claim than the evidence permits. The choices Roman conquest, Gunpowder warfare and Mongol invasion do not satisfy the same combination of site, feature, river region, crop, craft, discovery, chronology or cultural association. A distractor may be historically important in another context but still be wrong for the relationship asked here. Replaying that method confirms Changes in river systems as the keyed response for this Ancient History question. This evidence-led approach strengthens factual recall, chronological caution and option elimination for SSC, Railway, Banking, Defence, State and other competitive examinations.