An expression is a collection of?

The correct answer is D: Constants, variables and operators. An expression in programming (and mathematics) is a combination of one or more explicit values, variables, operators, and functions that the programming language interprets and computes to produce another value. This computed value is often used as part of a larger statement. For example, in x + 5 * y, x and y are variables, 5 is a constant, and + and * are operators, all forming a valid expression.

• A: Constants only is incorrect. While a constant (like 10 or "hello") can be a simple expression on its own, most meaningful expressions involve more than just constants to perform calculations or comparisons.
• B: Variables only is incorrect. Similar to constants, a variable (like age) can be a simple expression, but it typically doesn't perform an operation unless combined with operators or other elements. Complex expressions are built from multiple components.
• C: Operators only is incorrect. Operators (like +, -, *, /, ==) define operations, but they require operands (constants or variables) to act upon. An operator by itself, without any values to operate on, does not form a complete expression. For instance, just writing + is not a valid expression that can be evaluated.