In the C++ standard library, within which namespace are the standard exception classes, such as std::exception and std::runtime_error, defined?

Introduction / Context:
The C++ standard library groups its classes and functions into namespaces to avoid name clashes and to organise components logically. Exception classes that represent common error conditions are part of this standard library. This question asks in which namespace these standard exception classes are defined.

Given Data / Assumptions:

• We are talking about standard exception classes such as std::exception, std::logic_error and std::runtime_error.
• These classes are defined by the C++ standard library, not by user code.
• They must live in a well known namespace so that programmers can refer to them consistently.

Concept / Approach:
All standard C++ library components are defined in the namespace std. This includes containers, algorithms, strings, streams and exception classes. The use of std as a namespace keeps the global namespace clean and allows user code to define its own classes without conflicting with standard names. To reference standard exceptions, you either qualify them with std:: or bring them into scope with a using declaration, although fully qualified names are often preferred in header files to avoid pollution.

Step-by-Step Solution:
Step 1: Recall that the C++ standard library is contained in namespace std.Step 2: Recognise that exception classes like exception and runtime_error are part of that library.Step 3: Therefore they are also defined in namespace std and should be written as std::exception and std::runtime_error.Step 4: Option A explicitly states namespace std.Step 5: Other options suggest fictional namespaces or the global namespace, so option A is correct.

Verification / Alternative check:
If you inspect standard library headers such as <exception> or <stdexcept>, you will see declarations similar to namespace std { class exception; class runtime_error; }. This confirms that these classes are defined within the std namespace. Code examples in documentation also show usage like throw std::runtime_error("message");.

Why Other Options Are Wrong:
Option B and option C invent namespaces that are not part of the standard. Option D claims that exceptions live in the global namespace, which would conflict with the convention that all standard library components are under std. Option E introduces another non standard namespace. None of these match the language specification.

Common Pitfalls:
Beginners sometimes omit the std:: prefix and rely heavily on using namespace std; in global scope, which can lead to name clashes in larger projects. While this does not change where the classes are defined, it can cause confusion. In exam questions, however, you simply need to remember that standard exceptions are in the std namespace.

Final Answer:
namespace std