Section: Software Development Security

Bell LaPadula model (Bell 1975): The granularity of objects and subjects is not predefined, but the model prescribes simple access rights. Based on simple access restrictions the Bell LaPadula model enforces a discretionary access control policy enhanced with mandatory rules. Applications with rigid confidentiality requirements and without strong integrity requirements may properly be modeled.
These simple rights combined with the mandatory rules of the policy considerably restrict the spectrum of applications which can be appropriately modeled.
Source: TIPTON, Hal, (ISC)2, Introduction to the CISSP Exam presentation.
Also check:
Proceedings of the IFIP TC11 12th International Conference on Information Security,
Samos (Greece), May 1996, On Security Models.

In computer science, hierarchical protection domains, often called protection rings, are a mechanism to protect data and functionality from faults (fault tolerance) and malicious behavior (computer security). This approach is diametrically opposite to that of capability-based security.
Computer operating systems provide different levels of access to resources. A protection ring is one of two or more hierarchical levels or layers of privilege within the architecture of a computer system. This is generally hardware-enforced by some CPU architectures that provide different CPU modes at the hardware or microcode level. Rings are arranged in a hierarchy from most privileged (most trusted, usually numbered zero) to least privileged (least trusted, usually with the highest ring number). On most operating systems, Ring 0 is the level with the most privileges and interacts most directly with the physical hardware such as the CPU and memory.
Special gates between rings are provided to allow an outer ring to access an inner ring's resources in a predefined manner, as opposed to allowing arbitrary usage. Correctly gating access between rings can improve security by preventing programs from one ring or privilege level from misusing resources intended for programs in another. For example, spyware running as a user program in Ring 3 should be prevented from turning on a web camera without informing the user, since hardware access should be a Ring 1 function reserved for device drivers. Programs such as web browsers running in higher numbered rings must request access to the network, a resource restricted to a lower numbered ring.
Ring Architecture
All of the other answers are incorrect because they are detractors.
References:
OIG CBK Security Architecture and Models (page 311)
and
https://en.wikipedia.org/wiki/Ring_%28computer_security%29

Explanation/Reference:
Explanation:
The trusted computing base (TCB) is a collection of all the hardware, software, and firmware components within a system that provide some type of security and enforce the system's security policy. The TCB does not address only operating system components, because a computer system is not made up of only an operating system. Hardware, software components, and firmware components can affect the system in a negative or positive manner, and each has a responsibility to support and enforce the security policy of that particular system. Some components and mechanisms have direct responsibilities in supporting the security policy, such as firmware that will not let a user boot a computer from a USB drive, or the memory manager that will not let processes overwrite other processes' data. Then there are components that do not enforce the security policy but must behave properly and not violate the trust of a system. Examples of the ways in which a component could violate the system's security policy include an application that is allowed to make a direct call to a piece of hardware instead of using the proper system calls through the operating system, a process that is allowed to read data outside of its approved memory space, or a piece of software that does not properly release resources after use.
To assist with the evaluation of secure products, TCSEC introduced the idea of the Trusted Computing Base (TCB) into product evaluation. In essence, TCSEC starts with the principle that there are some functions that simply must be working correctly for security to be possible and consistently enforced in a computing system. For example, the ability to define subjects and objects and the ability to distinguish between them is so fundamental that no system could be secure without it. The TCB then are these fundamental controls implemented in a given system, whether that is in hardware, software, or firmware.
Each of the TCSEC levels describes a different set of fundamental functions that must be in place to be certified to that level.
Incorrect Answers:
A: Software is also considered part of the Trusted Computing Base.
B: Firmware is also considered part of the Trusted Computing Base.
D: Trusted computer operators and system managers are not considered part of the Trusted Computing Base.
References:
Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, New York, 2013, p. 360
https://www.freepracticetests.org/documents/TCB.pdf