Typically the decision on what should be the lowest level of CI recorded is influenced mostly by:

The fundamental purpose of Configuration Management (CM) is to establish and maintain control over a product's attributes throughout its lifecycle. A Configuration Item (CI) is an entity within the system that is designated for separate configuration control. The decision on the lowest level of CI to record is primarily driven by the need to manage changes independently. If a component is expected to be designed, procured, tested, repaired, or replaced as a distinct unit, it must be identified as a CI. This allows for precise tracking and control of its specific version and any modifications, ensuring the integrity of the overall system is maintained when individual parts are altered.

A. The reliability of the CIs: Reliability is an important characteristic of a component, but it does not determine its status as a CI. Both highly reliable and unreliable parts may be designated as CIs if they require independent change control.
C. The suitability of the available software to hold the information: The capabilities of a CM tool are a constraint, not the primary driver for CI selection. Engineering and management requirements should dictate what needs to be controlled, and the tools should be selected to support those needs.
D. The availability of spares for CIs: Spares management is a logistical function. While many CIs are also spare parts, the decision to designate an item as a CI is based on the need for formal change control, not simply its status as a spare.

1. ANSI/EIA-649-B, Configuration Management Standard. (2011). SAE International.

Section 5.2, "Configuration Identification": This section details the process of selecting CIs. It states, "CIs are selected for management visibility and control... Criteria for CI selection include, but are not limited to... [items that] will be separately designed, developed, procured, produced, tested, delivered, installed, operated, maintained, repaired, or retired." This directly supports the principle that the level of independent change, maintenance, and repair dictates CI selection.

2. Tahan, M., & Rausand, M. (2012). Configuration Management: Theory and Application for Engineers, Managers, and Students. John Wiley & Sons.

Chapter 4, "Configuration Identification," Section 4.3, "Selection of Configuration Items": The text explains that a key criterion for selecting a CI is that it is a "replaceable unit." It elaborates that the "level of decomposition depends on the complexity of the product and the level at which parts are expected to be replaced during the operational and support phase." (p. 63). This aligns directly with the chosen answer.

3. Blanchard, B. S. (2004). System Engineering Management (3rd ed.). John Wiley & Sons.

Chapter 11, "Configuration Management": This chapter discusses that CIs are selected based on the need for formal control. It notes that CIs are typically items "subject to replacement" and that the breakdown into CIs should reflect the "logistics support structure," which is based on which components will be independently replaced or repaired. (pp. 268-270).