A logical expression is a statement that evaluates to a Boolean value, either true or false. The expression z >= 6 is a comparison that checks if the value of variable z is greater than or equal to 6. The result of this comparison will be true or false, making it a logical expression. Such expressions are fundamental for controlling program flow in conditional statements and loops.

A. z = 7: This is an assignment expression, which assigns the value 7 to the variable z. It does not evaluate to a Boolean.

B. 6 + 7: This is an arithmetic expression that evaluates to a numeric value (13), not a Boolean.

D. "6" + "7": This is a string concatenation expression that evaluates to a string value ("67"), not a Boolean.

1. Mozilla Developer Network (MDN). (2023). Expressions and operators. In JavaScript Guide. Section: "Comparison operators". Retrieved from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/ExpressionsandOperators#comparisonoperators

2. Haverbeke, M. (2018). Eloquent JavaScript (3rd ed.). No Starch Press. Chapter 1: "Values, Types, and Operators", Section: "Boolean Values".

3. Flanagan, D. (2020). JavaScript: The Definitive Guide (7th ed.). O'Reilly Media. Chapter 4: "Expressions and Operators", Section 4.9 "Relational Operators".