In modern C programming, will the following function definitions compile and work correctly as written, and why? f1(int a, int b) { return (f2(20)); } f2(int a) { return (a * a); }

Introduction / Context:
This question examines how C handles function declarations, return types, and call order. In older C dialects it was sometimes legal to omit explicit return types, and the compiler would assume an int return type by default. However, modern C standards require explicit return type declarations and discourage implicit assumptions. The example also demonstrates the importance of providing a prototype or earlier definition for a function like f2 before it is called by another function such as f1.

Given Data / Assumptions:
- The functions f1 and f2 are written without explicit return types before their names.
- f1 calls f2 with the argument 20 and attempts to return the value produced by f2.
- f2 takes an int parameter and returns the product a * a.
- There is no prior prototype for f2 before its use inside f1.
- We assume a modern C compiler that follows current language rules rather than very old implicit int behaviours.

Concept / Approach:
Under current C standards, every function definition must include an explicit return type. Omitting the return type is not allowed, and relying on the compiler to guess a type is considered an error. Additionally, if one function calls another, the compiler needs to know the callee signature at the point of call, which is achieved by placing a prototype or full definition before the call. Without this, the compiler cannot reliably perform type checking. Therefore, the given code as written is not valid modern C and needs corrections to compile correctly.

Step-by-Step Solution:
1. Examine the definition of f1: f1(int a, int b) { return (f2(20)); }. There is no explicit return type specified before f1. 2. Examine the definition of f2: f2(int a) { return (a * a); }. Similarly, there is no explicit return type before f2. 3. In modern C, both functions must have a declared return type, such as int f1(int a, int b) and int f2(int a). 4. At the point where f1 is compiled, the definition of f2 appears later in the source, and there is no preceding prototype telling the compiler about f2. 5. Without a prototype and with missing return types, the compiler cannot correctly apply the language rules and will report errors. 6. The correct approach is to write int f2(int a); before f1, and then define int f1(int a, int b) { return f2(20); } and int f2(int a) { return a * a; }.

Verification / Alternative check:
If you add explicit return types and a prototype, the code becomes straightforward: f2 squares its argument, and f1 always returns f2(20), which is 400. Compiling the original code without return types and prototypes on a strict compiler will result in errors rather than just warnings. This comparison confirms that the problem lies in missing type information and the call order rather than in the idea of one function calling another.

Why Other Options Are Wrong:
Option B is incorrect because in modern C the code will not always just work; missing return types and prototypes are not acceptable practice and often cause compilation failure. Option C is wrong because C fully supports calling one function from another; this is a basic feature of the language. Option D is misleading because the correctness of the code does not depend on the sign of the parameters. The real issue is the lack of explicit types and prototypes, as described in option A.

Common Pitfalls:
A typical mistake is relying on outdated examples that omit return types and assuming they are still correct today. Another pitfall is believing that the compiler will always infer the right types automatically, which may hide subtle bugs or cause undefined behaviour. Developers sometimes forget that function prototypes are essential for type checking, especially when compiling with strict warning levels or using separate compilation units. This question reinforces the habit of always declaring clear function signatures and placing prototypes before use.

Final Answer:
As written, the functions are missing explicit return types and lack a prototype for f2 before its use, so the code is not correct in modern C. The correct choice is no, because both functions are missing an explicit return type and there is no prototype for f2 before it is called.