The correct answer is option 'A', Mohenjodaro.

Explanation:
Mohenjodaro is one of the most famous Harappan sites in the Indus Valley Civilization, which existed around 2500 BCE to 1900 BCE. It is located in modern-day Pakistan.

1. Discovery of Human Skeletons: The discovery of human skeletons in the upper levels of the site indicates that there was some form of violent event that took place at Mohenjodaro.

2. Skeletons Lying Pell-Mell: The skeletons found at Mohenjodaro were found in a disordered or chaotic manner, lying pell-mell. This suggests that these individuals were not buried with proper rituals or care, indicating a sudden and violent event.

3. Skull with Injury Mark: Among the human remains, a skull with an injury mark was found. This injury mark further supports the theory of an invasion and massacre at Mohenjodaro. The presence of such an injury indicates that the individual suffered a violent death, possibly as a result of warfare or conflict.

4. Invasion and Massacre: Based on the evidence of the disordered skeletons and the presence of a skull with an injury mark, it can be inferred that Mohenjodaro was invaded and a massacre took place. The violent nature of the event suggests that there was a significant conflict or attack on the site.

5. Indication of Decline: The invasion and massacre at Mohenjodaro provide insights into the decline of the Indus Valley Civilization. While the exact reasons for the decline are still debated among scholars, evidence of violent events like this suggests that external factors such as invasions may have played a role.

In conclusion, the discovery of human skeletons lying pell-mell with a skull having an injury mark at Mohenjodaro provides evidence of an invasion and massacre at the site. This discovery adds to our understanding of the violent events that took place during the decline of the Indus Valley Civilization.

The correct answer is mohenjodaro. it is also known as "Mound of the deads" because there was the discovery of the skeleton, and from this site, the archaeologists found a lot of skeletons. which shows the massacre and invasions (most probably of the aryans) in the upper level.