Which expression represents a one-variable linear equation?

• A. ax + by + c = 0
• B. y = mx + b
• C. ax^2 + b = 0
• D. ax + b = 0 where a ≠ 0

Answer: (D)

A one-variable linear equation is an equation that involves exactly one unknown and is linear in that unknown. The standard form is ax + b = 0 with a ≠ 0, because it shows a single variable x raised to the first power, plus a constant, equal to zero.

That makes the expression ax + b = 0 the best fit. It has only one variable and the x-term is linear. The other options either involve two variables (x and y), express a relationship between two variables rather than solve for one variable, or have a squared term (x^2), which makes them nonlinear.