In C++ object oriented programming, which of the following is not a valid access specifier keyword (public, protected, or private)?

Introduction / Context:
Access control is a core concept in C++ object oriented programming. It allows a class designer to decide which parts of a class are visible to other classes or to external code. The C++ language provides a small, fixed set of access specifiers that control whether members are accessible publicly, only by derived classes, or only inside the class itself. This question checks whether you remember exactly which keywords are valid access specifiers and can therefore distinguish them from other C++ keywords that have completely different roles.

Given Data / Assumptions:

• The language is standard C++.
• We are talking specifically about access specifiers for class members.
• The commonly used access specifiers in C++ are public, protected, and private.
• Any other keyword listed alongside these may be a distractor and not an access specifier.

Concept / Approach:
In C++, the three valid access specifier keywords are public, protected, and private. These keywords appear in class or struct definitions followed by a colon and apply to the members declared after them. Public members are accessible from anywhere the class is visible. Protected members are accessible inside the class itself and by derived classes, but not by unrelated external code. Private members are accessible only inside the class and by designated friends. The keyword default does exist in C and C++ but it is used in switch statements, not as an access specifier. Any answer that suggests default as an access specifier is therefore incorrect.

Step-by-Step Solution:
Step 1: List the standard C++ access specifiers: public, protected, and private. Step 2: Compare each option with this list. Public appears in the list, so it is a valid access specifier. Step 3: Protected also appears in the list, so it is a valid access specifier. Step 4: Private appears in the list as well and is therefore another valid access specifier. Step 5: Default is not in the list of access specifiers and is instead used in switch statements, so it is not an access specifier.

Verification / Alternative check:
If you recall the typical layout of a class, you will remember code such as class Example { public: int x; private: int y; protected: void helper(); }. There is no place in this syntax where default appears. Similarly, in documentation and tutorials about C++ access control, only public, protected, and private are described. This consistent absence of default in the context of member access control confirms that it is not a valid access specifier keyword.

Why Other Options Are Wrong:
Option public: This is a valid access specifier and cannot be the correct choice for a question asking which one is not an access specifier. Option protected: This is also a valid access specifier that controls access for derived classes and is therefore not the right answer. Option private: This is the default access specifier for classes and is a central part of encapsulation, so it is clearly valid.

Common Pitfalls:
Many learners confuse the concept of a default access level with a keyword named default. In C++, classes default to private access for members if no specifier is given, and structs default to public, but there is no keyword default used to declare this. Another pitfall is mixing up the default label in a switch statement with access control keywords. Keeping in mind that switch uses the case and default labels, while classes use public, protected, and private, helps avoid this confusion.

Final Answer:
The option that is not a valid C++ access specifier is default.