In Java, what are class variables, member (instance) variables, and local variables, and how do they differ in scope and lifetime?

Introduction / Context:
Java uses different kinds of variables to represent data at the class level, object level, and method level. Understanding the distinction between class variables, member variables, and local variables is important for reasoning about memory, scope, and side effects in Java programs. This question asks you to identify these categories and explain how they differ.

Given Data / Assumptions:

• We are working in standard Java with classes and objects.
• A class can declare static fields and non static fields.
• Methods can declare variables inside their bodies, including method parameters and block variables.
• The programmer must manage these different scopes correctly to avoid bugs.

Concept / Approach:
Class variables are declared with the static keyword inside a class but outside any method. They are associated with the class itself and shared across all instances of that class. Member or instance variables are declared inside a class but without static, and each object created from the class has its own copy. Local variables are declared inside methods, constructors, or blocks such as loops and if statements; they exist only while the method or block is executing and are not visible outside their scope.

Step-by-Step Solution:
Step 1: Recognize that a declaration like static int counter; inside a class is a class variable, shared by every object of that type.Step 2: A declaration like int age; inside the same class but outside methods is a member or instance variable; every new object of that class gets its own age field.Step 3: Inside a method, a line like int i = 0; declares a local variable that exists only while the method is running and is not shared between calls.Step 4: Method parameters are also local variables in the sense that they have method scope and disappear when the method returns.Step 5: Option A correctly describes the scope and sharing properties of class, member, and local variables, so it is the correct choice.

Verification / Alternative check:
You can verify these differences by creating several objects of the same class and observing their behaviour. Changing a static field through one object affects the value seen by all other instances, proving that class variables are shared. By contrast, changing a member variable affects only that specific object. Local variables inside methods cannot be accessed from outside the method and do not retain values between calls, which can be confirmed by simple test programs.

Why Other Options Are Wrong:
Option B incorrectly ties access modifiers to variable categories, which is not how Java works. Option C claims that class and member variables are the same, ignoring the distinction between static and instance fields. Option D reverses how variables relate to threads; local variables are usually thread safe because each thread has its own call stack, while class and member variables can be shared across threads and need synchronization for safe access.

Common Pitfalls:
A common mistake is using static where an instance field is intended, which can cause unexpected sharing of data between objects. Another pitfall is relying on uninitialized local variables; Java requires that local variables be explicitly assigned before use, unlike fields that receive default values. Understanding the scope and lifetime of each variable category helps prevent subtle bugs related to state and concurrency.

Final Answer:
Class variables are static fields shared by all instances, member variables are instance fields stored separately in each object, and local variables are declared inside methods or blocks and exist only during that execution.