In C plus plus function overloading, which of the following aspects cannot be used alone to distinguish overloaded functions and therefore cannot be the sole basis for overloading?

Introduction / Context:
Function overloading in C plus plus allows you to have multiple functions with the same name but different parameter lists. The compiler uses the parameter types and counts to determine which version to call. However, certain aspects of a function, such as its return type, do not participate in overload resolution. This question asks which aspect cannot be used alone to distinguish overloaded functions, meaning that relying on it would cause a compilation error.

Given Data / Assumptions:

• We are working in C plus plus with normal function declarations.
• Overloading is based on a function's signature as seen by the compiler.
• The parameter list forms the basis of overload resolution.
• The return type is not considered when deciding which overload to call.

Concept / Approach:
In C plus plus, two functions can be overloaded if they differ in the number or types of their parameters or, in some cases, the order of parameters with different types. The return type alone is not part of the function signature for overloading purposes. If you attempt to define two functions with the same name and identical parameter lists but different return types, the compiler will treat this as a redefinition or ambiguous declaration, not as a valid overload.

Step-by-Step Solution:
Step 1: Recall the rule that the function signature for overloading includes the function name and parameter types, not the return type. Step 2: Recognize that changing only the return type while leaving the parameter list unchanged does not create a distinct overload in the eyes of the compiler. Step 3: Examine option a, which describes changing only the return type with the same parameter list, which is not allowed as a sole basis for overloading. Step 4: Note that options b, c, and d involve changes to the parameter list in valid ways for overloading. Step 5: Conclude that option a is the correct answer.

Verification / Alternative check:
If you write C plus plus code such as int func(int x); and double func(int x); in the same scope, the compiler reports an error about redefinition or conflicting types. In contrast, defining int func(int x); and int func(double x); compiles correctly and allows overloading based on parameter type. This behavior confirms that the return type alone cannot distinguish overloads.

Why Other Options Are Wrong:
Option b, changing the number of parameters, is a classic way to overload functions. Option c, changing parameter types, is also valid. Option d, changing the order of parameters when they have different types, can create distinct overloads. Option e describes a mix of default arguments and parameter type changes, which still relies on differences in parameter lists, not return type alone.

Common Pitfalls:
New C plus plus programmers sometimes attempt to overload functions by return type alone to represent different kinds of results, which leads to compile time errors. Another pitfall is to combine default arguments with overloading in a way that creates ambiguous calls. Always design overloads based on clear and distinct parameter signatures rather than relying on return types.

Final Answer:
You cannot overload functions solely by changing their return type, so changing only the return type while keeping the same parameter list cannot be used as the basis for overloading, as stated in option a.