Step 1: Understanding the Concept:
The question requires arranging some of India's most well-known financial scams in the chronological order of their public exposure.
Step 2: Detailed Explanation:
Let's establish the year each scam came to light:
A. Harshad Mehta Scam: This was a major stock market manipulation scam that was exposed in 1992.
D. Ketan Parekh Scam: This was another significant stock market scam, orchestrated by a protege of Harshad Mehta, which was unearthed in 2001.
B. Satyam Scam: This was a corporate fraud involving the manipulation of company accounts by its founder. The scandal broke in January 2009.
C. PNB Nirav Modi Scam: This was a large-scale fraud involving the issuance of fraudulent Letters of Undertaking at Punjab National Bank. It came to public attention in early 2018.
Step 3: Final Answer:
Arranging these scams from the earliest to the latest based on the year of exposure:
1. Harshad Mehta Scam (1992) $\rightarrow$ A
2. Ketan Parekh Scam (2001) $\rightarrow$ D
3. Satyam Scam (2009) $\rightarrow$ B
4. PNB Nirav Modi Scam (2018) $\rightarrow$ C
The correct sequence is A, D, B, C.