Final answer:
An algebraic expression is considered undefined when a specific variable substitution results in an expression that does not produce a real or imaginary answer, such as dividing by zero.
Step-by-step explanation:
An algebraic expression is called undefined when substituting a specific value for one of its variables results in no answer. This occurs, for example, when you attempt to divide by zero or when the value you substitute creates a situation that is not mathematically possible, such as taking the square root of a negative number in real numbers.
To understand this concept through the context of linear equations, all of the options given in the practice test question (A. y = -3x, B. y = 0.2 +0.74x, and C. y=-9.4 - 2x) are linear because they can be written in the form of y = mx + b, where m and b are constants.
This form is defined for all real values of 'x', and does not lead to undefined expressions unless specific domain restrictions apply.
The correct option is D) Undefined