Version control is a fundamental component of Software Configuration Management (SCM). The primary goal of SCM is to establish and maintain the integrity of a system's artifacts throughout its lifecycle. These artifacts, known as Configuration Items (CIs), are not limited to source code. They encompass all items necessary to define, build, test, and deploy the system. This includes project plans, process documents like procedures, and critical environment components. The specific operating system version and its configuration are crucial for ensuring consistency and reproducibility across development, testing, and production environments. Therefore, all listed items should be managed under a version control system.

A, B, C, D: These options are individually correct items to be placed under version control, but they are incomplete. The most comprehensive answer includes all of these critical project artifacts.

1. IEEE Std 828-2012, IEEE Standard for Configuration Management in Systems and Software Engineering.

Section 5.3, "Configuration identification," describes the selection of CIs. It states that CIs can include "documentation (e.g., plans, specifications, and manuals)" and "system-related life cycle data (e.g., requirements data, design data, test data, and environment data)." This standard directly supports placing code, plans, procedures, and operating systems (as environment data) under configuration management.

2. Bersoff, E. H. (1984). Elements of Software Configuration Management. IEEE Transactions on Software Engineering, SE-10(1), 79-87.

On page 80, this foundational academic paper defines software configuration items as including "computer programs (as source code, object code), documents that describe such programs (e.g., requirements, design, test documentation), and data (contained within the programs or external to them)." This covers code (A), procedures (B), and plans (C).

3. MIT OpenCourseWare. (2018). 6.033 Computer System Engineering, Spring 2018. Massachusetts Institute of Technology.

Lecture 2 notes on "The Software Lifecycle" discuss configuration management and the need to version not just code but also "build instructions, documentation, test suites, and other artifacts." The principle of a reproducible build environment, a core tenet of SCM, necessitates controlling the operating system and other environmental dependencies.