Encryption in transit is the process of protecting data from unauthorized access or modification while it is being transferred from one location to another, such as from an on-premises data center to a cloud service provider. Encryption in transit uses cryptographic techniques to scramble the data and make it unreadable to anyone who intercepts it, except for the intended recipient who has the key to decrypt it. Encryption in transit is one of the best approaches to optimize data security in an IaaS migration, as it reduces the risk of data breaches, tampering, or leakage during the data transfer. Encryption in transit can be implemented using various methods, such as Transport Layer Security (TLS), Secure Sockets Layer (SSL), Internet Protocol Security (IPsec), or Secure Shell (SSH).

Encryption in transit is different from other options, such as reviewing the risk register, performing a vulnerability scan, or performing server hardening. Reviewing the risk register is the process of identifying, analyzing, and prioritizing the potential threats and impacts to the data and the cloud environment. Performing a vulnerability scan is the process of detecting and assessing the weaknesses or flaws in the data and the cloud infrastructure that could be exploited by attackers. Performing server hardening is the process of applying security measures and configurations to the cloud servers to reduce their attack surface and improve their resilience. While these options are also important for data security, they do not directly address the dataprotection during the migration process, which is the focus of the question. References: What is encryption in transit? - Definition from WhatIs.com, Data Encryption in Transit Guidelines - UC Berkeley Security, Cloud Computing Security - CompTIA Cloud Essentials+ (CLO-002) Cert Guide