Explanation:

A programmer is an individual whose primary role involves writing, developing, and testing computer programs. They translate design specifications into executable code using various programming languages and ensure the code functions correctly through debugging and testing. While a software engineer (c) also writes and tests programs, their role is generally broader, encompassing the entire software development lifecycle, including design, architecture, and project management. Computer scientists (b) focus more on the theoretical foundations of computing. "Project developer" (d) is a more general term that could encompass various roles within a project. Therefore, "Programmer" is the most direct and specific term for individuals primarily engaged in writing and testing programs.