Attestation refers to providing formal evidence or proof that a particular process or activity has been completed according to standards or requirements. In this context, attestation involves demonstrating to customers or stakeholders that software developers have received appropriate training on secure coding practices.
Assurance generally refers to confidence or guarantees about the security posture but does not specifically mean proving or certifying training. Due diligence is the effort made to ensure compliance or safety, but it is not the act of proving training has occurred. A contract is a legal agreement, which may include requirements for training but is not the act of proving training itself.
The importance of attestation in compliance and governance processes is discussed in the Security Program Management and Oversight domain in SY0-701 materials#7:Chapter 5 CompTIA Security+ Practice Tests#.