In a traditional Java interface (before default and private methods), which of the following method declarations is allowed inside the interface?

Introduction / Context:
Classic Java interfaces, as used in many exam questions, originally allowed only abstract method declarations without bodies. This question focuses on which method signatures are valid inside such an interface and reinforces the rule that interface methods are implicitly public and abstract unless newer Java features like default or static methods with bodies are introduced.

Given Data / Assumptions:

• We are considering a traditional Java interface model, ignoring Java 8 default methods and later additions.
• Methods declared in an interface do not have bodies and act as abstract contracts.
• Interface methods are implicitly public and abstract, even if those keywords are omitted.
• Access modifiers such as private and protected were not allowed on interface methods in older Java versions.

Concept / Approach:
In this classic model, an interface method declaration must be compatible with public abstract semantics and must not include a method body. Writing public abstract void m1(); clearly matches this pattern. Writing a method with a body inside an interface, such as public void m1() { } in the pre Java 8 model, is not allowed. Likewise, using private or protected as access modifiers on interface methods is not valid in that traditional context, because interface methods must be publicly accessible to implementing classes.

Step-by-Step Solution:
Step 1: Examine option A, public void m1() { }. In a classic interface, this is invalid because it provides a method body where only abstract signatures are allowed.Step 2: Examine option B, public abstract void m1(); which is a valid abstract method declaration with no body and correct access level.Step 3: Examine option C, private void m1(); which conflicts with the requirement that interface methods must be accessible to implementing classes, so it is not allowed in this traditional model.Step 4: Examine option D, protected void m1(); which again breaks the public accessibility rule for interface methods.Step 5: Conclude that only option B is valid for a classic Java interface method and therefore is the correct answer.

Verification / Alternative check:
If you try to compile an interface that includes public void m1() { } in a Java version before default methods, the compiler reports an error that abstract methods cannot have a body. Similarly, using private or protected on interface methods results in compilation errors about illegal modifiers. In contrast, public abstract void m1(); compiles cleanly and is the standard way to declare an interface method signature in that model.

Why Other Options Are Wrong:
Option A uses a method body and therefore does not fit the abstract contract only style of traditional interfaces. Option C and option D use private and protected access, which are not permitted in classic interface method declarations because implementers must be able to call and override these methods publicly. Only option B respects both the abstract and access requirements.

Common Pitfalls:
One pitfall is mixing up modern Java features such as default and static interface methods with older exam style rules. Another is assuming that omitting abstract or public changes the nature of the method, when in fact interface methods are public and abstract by default. When answering interview questions, pay attention to whether the context refers to older certification style rules or to modern Java capabilities.

Final Answer:
In a traditional Java interface only public abstract void m1(); is allowed, so option B is correct.