Which control structure is present in every program, regardless of size or complexity?

Introduction / Context:
Structured programming is built on three fundamental control structures: sequence, selection, and repetition. While many programs use conditional logic and loops, every program necessarily executes statements in some order—this ordering is the sequence structure.

Given Data / Assumptions:

• Any non-empty program has at least one statement to execute.
• Statements execute in a defined order unless control transfers (e.g., jumps, calls) alter the flow.
• Selection and repetition are optional depending on requirements.

Concept / Approach:
Sequence represents straightforward execution from one statement to the next. Even a single function that performs a calculation with no branches or loops is an instance of sequence. Therefore, sequence is universal, while selection (if/else) and repetition (while/for) appear only when needed.

Step-by-Step Solution:
1) Consider a minimal program with a single print or return—this is sequence. 2) Consider a numeric utility with no conditions—still sequence. 3) Recognize that selection and repetition are not mandatory in such cases. 4) Conclude that sequence is always present.

Verification / Alternative check:
Foundational CS literature defines these three as sufficient; of them, sequence is the irreducible baseline in any program execution.

Why Other Options Are Wrong:
Option A: Loops are optional.
Option B: Conditionals are optional.
Option D: “Switching” is a specific form of selection, not universally required.
Option E: Not applicable since one correct structure exists.

Common Pitfalls:
Assuming that all control structures are always present. Many scripts are purely sequential.

Final Answer:
Sequence