Final answer:
User-defined functions can be used in expressions similar to built-in functions. They can be included in expressions to calculate values, just like built-in functions.
Step-by-step explanation:
A user-defined function can be used in various contexts within a programming environment, similar to how a built-in function is used. The correct answer to the question 'A user-defined function can be used in ________ (like a built-in function).' is c) Expressions.
User-defined functions can indeed be called within loops, passed into arrays, included within scripts, but when considering where they can be equivalently used like built-in functions, it is within expressions that they are most versatile. Expressions are combinations of literals, operators, variables, and functions that are evaluated to produce another value. Both built-in and user-defined functions can be part of such expressions.
For example, if we define a function add(a, b), which sums two numbers, we can use it in an expression like c = add(5, 3) to assign the value of 8 to the variable c.