Step 1: Comprehension
The task is to arrange key historical events in Harappan archaeology chronologically, from earliest to latest.
Step 2: Elaboration
Event dates:
(A) Alexander Cunningham's Harappan Seal Report: Alexander Cunningham, the first Director-General of the ASI, reported on a Harappan seal he found in 1875. This is the earliest event.
(C) Daya Ram Sahni's Harappa Excavations: Systematic excavations at Harappa, leading to the discovery of the Indus Valley Civilization, commenced under Rai Bahadur Daya Ram Sahni in 1921. This is the subsequent significant event.
(B) S.R. Rao's Lothal Excavations: The excavation of the Harappan port city of Lothal in Gujarat was directed by S.R. Rao from 1955 to 1960.
(D) Vasant Shinde's Rakhigarhi Archaeogenetic Research: Archaeogenetic research, involving ancient DNA study, is a recent development. Significant research on Rakhigarhi skeletal remains by Vasant Shinde was published around 2018-2019, marking the most recent event.
Step 3: Conclusion
The chronological sequence is: (A) Cunningham's report, (C) Sahni's excavation, (B) Rao's excavation, and (D) Shinde's research. Thus, the order is (A), (C), (B), (D).