In structured programming, an algorithm that is written once and then reused from many different places in a program is commonly called what?

Introduction / Context:
One of the most important ideas in programming is code reuse. Instead of copying the same steps over and over again, developers encapsulate an algorithm in a single reusable unit and invoke it whenever needed. This question asks you to identify the common term used in many programming languages for such a reusable algorithm, especially in structured and procedural programming styles.

Given Data / Assumptions:

• The algorithm is written once in a single place in the source code.
• Different parts of the program can call or invoke this algorithm when they need its behaviour.
• The construct can optionally take parameters and may return a result.
• We are using the vocabulary of typical structured languages such as C, Pascal, or early versions of C plus plus.

Concept / Approach:
In most structured languages, a reusable block of code that can be invoked by name is called a function or procedure. A function usually returns a value, while a procedure may not, but many exam questions use the word function as the generic term. By packaging an algorithm into a function, the programmer can improve readability, reduce duplication, and make maintenance easier. Blocks by themselves may group statements but are not necessarily reusable by name. Data types and record separators are unrelated to the idea of calling the same algorithm from multiple places.

Step-by-Step Solution:
Step 1: Focus on the phrase written once and used in many places, which points to a named reusable construct.Step 2: Recall that functions and procedures are called by name and can be invoked from multiple locations within a program.Step 3: Recognise that a block is just a group of statements within braces or similar delimiters and may not be directly reusable from other scopes.Step 4: Note that data types describe kinds of values like integers or structures, not algorithms.Step 5: Conclude that the correct term for a reusable algorithm unit is a function in this context.

Verification / Alternative check:
Looking at C code, you will see functions such as printf or user defined functions, each providing a reusable behaviour. These functions are defined once but called many times. Tutorials on structured programming emphasise designing programs as a collection of functions, which reinforces the association between code reuse and functions. This matches the description given in the question stem.

Why Other Options Are Wrong:
Option A, block, refers to a grouping of statements, but a block without a name cannot be invoked from multiple places easily. Option C, data type, describes the form of data rather than behaviour. Option D, record separator, is a term from data storage or file formats and does not relate to reusable algorithms. None of these alternatives match the core idea of a named, reusable algorithm unit.

Common Pitfalls:
A common pitfall is confusing functions with objects or classes in object oriented languages. While methods in classes are also reusable algorithms, this question uses generic structured programming terminology, where function is the usual label. Another mistake is thinking that copying code manually gives reuse, but that approach increases maintenance cost. True reuse comes from centralising behaviour in functions or similar constructs, then calling them whenever needed.

Final Answer:
Correct answer: Function