Explanation:

- Option 1: Postal System

- Sher Shah Suri established a well-organized postal system to improve communication across his empire.

- It was one of his administrative innovations.

- Option 2: Spy System

- The spy system was not introduced by Sher Shah Suri for the first time.

- It existed in various forms before his rule.

- Option 3: Currency System

- He introduced a standard currency system with the issuance of the "Rupiya" which standardized coinage.

- This laid the foundation for the modern currency system in India.

- Option 4: Land Revenue System Based on Scientific Principles

- Sher Shah implemented a systematic land revenue assessment, which was based on land measurement and yield estimation.

- It was an innovative approach for its time.