Final answer:
A linear equation is an equation that can be written in the form y = mx + b, where m and b are constants.
Step-by-step explanation:
A linear equation is an equation that can be written in the form y = mx + b, where m and b are constants. In other words, a linear equation is an equation of a straight line.
In the given options, A and B are linear equations because they are in the form y = mx + b. Option C is not a linear equation because it has a term with x raised to a power of 2. Therefore, the correct answer is D, which includes both A and B.