Answer:
Equation D does not belong.
Explanation:
Equation A is a linear equation with variables x and y.
Equation B is a linear equation with variables c and t.
Equation C is a linear equation with variables a and b.
Equation D is a linear equation with a variable n.
Equations A, B, and C are equations with two variables.
Equation D has only one variable.
Equation D does not belong with the other three.