Final answer:
The logical expression (x && y && z) with x=true, y=false, and z=true evaluates to false because the logical AND operator requires all operands to be true. The second part of the question regarding HTML tags cannot be answered without additional information, but a valid HTML tag must be part of the HTML specification.
Step-by-step explanation:
If x=true; and y=false; and z=true; then what is if (x && y && z)? This expression uses the logical AND operator (&&), which returns true only if all operands are true. Since y is false in this case, the entire expression would evaluate to false.
Part 2 of the question asks: Which of the following is NOT a valid HTML tag? This question seems to be incomplete as it does not provide the options for HTML tags to be evaluated. However, it is worth noting that valid HTML tags are predefined in the HTML specification, and any tag not defined in this specification is considered invalid.