The decline of the Harappan civilization was a complex process involving multiple factors. The correct answer is option 'D' – all of these factors played a role in its decline. Let's explore each factor in detail:

Climatic Change:
- One of the significant factors that contributed to the decline of the Harappan civilization was climatic change.
- The region experienced a shift in climate patterns, with a decrease in rainfall and an increase in aridity.
- This led to a reduction in the flow of major rivers like the Indus, making agricultural activities challenging and affecting the availability of water for irrigation.
- As a result, the agricultural productivity of the region declined, leading to food scarcity and economic instability.

Floods:
- Another factor that contributed to the decline of the Harappan civilization was floods.
- The Indus River and its tributaries, which were essential for agricultural activities, experienced periodic flooding.
- However, towards the later stages of the civilization, the frequency and intensity of floods increased.
- These floods caused damage to infrastructure, including the destruction of irrigation systems and agricultural fields.
- The inability to manage and recover from these floods further weakened the civilization.

Deforestation:
- Deforestation is another factor that played a role in the decline of the Harappan civilization.
- The civilization relied heavily on timber for various purposes such as construction, fuel, and pottery production.
- The extensive deforestation of the region led to environmental degradation, soil erosion, and loss of biodiversity.
- This, in turn, affected agricultural productivity and contributed to the decline of the civilization.

Conclusion:
- In conclusion, the decline of the Harappan civilization was not solely caused by a single factor, but rather a combination of multiple factors.
- Climatic change, floods, and deforestation all played a role in the decline.
- The interplay between these factors resulted in economic instability, food scarcity, environmental degradation, and the eventual collapse of the Harappan civilization.