If the same header file is included twice in a C project, will it always cause an error, or can compilation still succeed? Choose the most accurate statement about multiple inclusion and header guards.

Introduction / Context:
Headers are often included in multiple translation units or indirectly through other headers. This question probes understanding of include guards (#ifndef/#define/#endif) and their role in avoiding redefinition errors.

Given Data / Assumptions:

• We consider two cases: headers with guards and headers without guards.
• Compilers/platforms can differ in diagnostics and tolerances, but the language rules are consistent.

Concept / Approach:
Multiple inclusion does not inherently break compilation. If a header employs guards (or #pragma once) and only contains repeatable declarations, double inclusion is harmless. Without guards, duplicate definitions (for example, non-static objects or functions with multiple definitions) can cause errors. Some compilers diagnose more aggressively; others may allow benign redeclarations.

Step-by-Step Solution:
Case 1: Guarded header → compiling succeeds even if included twice.Case 2: Unguarded, only declarations → may still compile, but fragile.Case 3: Unguarded with definitions → likely multiple-definition error at compile or link time.

Verification / Alternative check:
Add #ifndef H_H / #define H_H / #endif to a header; include it twice and confirm a single effective inclusion.

Why Other Options Are Wrong:
“Always yes” and “Always no” overgeneralize. Platform-limited claims (Linux or system headers only) are incorrect.

Common Pitfalls:
Placing non-inline function definitions or global objects in headers without guards; forgetting that #pragma once is non-standard but widely supported.

Final Answer:
It is compiler dependent.