A linear equation is an equation in which the degree of all the variables involved is 1.
Suppose I want an equation in two variables, say x and y, to be linear, I must have an equation in which the powers of x and y are 1. So, the general form would be ax+by=c where a,b and c are constants. You can extend this concept to many variables as well.
Similarly, quadratic equations are equations which involve degree 2 variables, cubic equations are equations which involve degree 3 variables and so on.
Option (b) is the right answer.