When implementing components of the C++ standard library, which of the following design practices are generally recommended?

Introduction / Context:
The C++ standard library sets a style and quality benchmark for user code. Many guidelines have emerged from how the library is written, including naming conventions, return value choices and the use of swap functions. This question asks you to identify which combination of practices is generally considered good style when implementing standard like library components.

Given Data / Assumptions:

• We are thinking about how standard library classes and functions are designed.
• Public names must follow rules that avoid conflict with user code and implementation details.
• Returning objects by value and providing swap functions are important design decisions.

Concept / Approach:
Public names in the standard library avoid trailing underscores, which are often reserved for implementation details or internal identifiers. Returning complex objects by value is common in modern C++, especially when compilers can apply return value optimisation and move semantics to make this efficient. Providing a member swap function can improve performance and exception safety, because it allows algorithms such as std::swap to exchange resources with minimal overhead. Taken together, these practices reflect a design philosophy of clarity, efficiency and compatibility with generic algorithms.

Step-by-Step Solution:
Step 1: Evaluate option A, which relates to naming conventions; avoiding trailing underscores on public names is consistent with common style.Step 2: Evaluate option B, where returning objects by value is widely used in modern C++ to express value semantics and leverage optimisations.Step 3: Evaluate option C, where providing member swap functions is known to help with performance and exception safety.Step 4: Since all three individual practices are reasonable and encouraged in standard like code, a combined option stating that all are recommended should be correct.Step 5: Option D explicitly states that all of the mentioned practices are recommended, so it is the correct answer.

Verification / Alternative check:
Looking at standard library types such as std::string or std::vector, you will see that they have clear public names without trailing underscores, are often returned by value from functions and provide member swap functions. These real examples support the idea that the practices named in options A, B and C align with standard library implementation style.

Why Other Options Are Wrong:
Options A, B and C each capture only one aspect of good practice and do not fully answer the question that asks for the overall set of recommended design practices. Option E claims that none of the mentioned practices should be used, which contradicts how the standard library is actually written.

Common Pitfalls:
Some developers overuse underscores or internal naming conventions in their public interfaces, which can confuse users. Others avoid returning objects by value due to outdated worries about performance, even though modern compilers optimise such code well. Forgetting to provide swap can also make generic algorithms less efficient. For exam purposes, remembering that all three practices are recommended will help you choose option D.

Final Answer:
All of the mentioned design practices are generally recommended for coding standard C++ library components.