in a controlled application development environment, the MOST important segregation of duties should be between...

 In a controlled application development environment, the most important segregation of duties should be between the person who implements changes into the production environment and the application programmer. This segregation of duties ensures that no one person can create and deploy code without proper review, testing, and approval. This reduces the risk of errors, fraud, or malicious code being introduced into the production environment.

The other options are not as important as the segregation between the application programmer and the person who implements changes into production, but they are still relevant for achieving a secure and reliable application development environment. The segregation of duties between the person who implements changes into production and the systems programmer is important to prevent unauthorized or untested changes to system software or configuration. The segregation of duties between the person who implements changes into production and the computer operator is important to prevent unauthorized or uncontrolled access to production data or resources. The segregation of duties between the person who implements changes into production and the quality assurance (QA) personnel is important to ensure independent verification and validation of code quality and functionality.

References:

ISACA CISA Review Manual 27th Edition (2019), page 247

Segregation of Duties in an Agile Environment | AKF Partners3

Separation of Duties: How to Conform in a DevOps World4