[tex]\qquad\qquad\huge\underline{{\sf Answer}}[/tex]
Linear equation has a degree of 1, that means the maximum power of a variable in a linear equation/expression should be 1. if an equation doesn't satisfy this condition then it's not a linear equation.
So, by Observing the choices, we get to the conclusion that the one which isn't a linear equation is B
[tex]\qquad \sf \dashrightarrow \: {x}^{3} - 5 {y}^{2} = 4[/tex]
Here, the variables have greater powers tham one, and degree is 3. so it's the required Answer ~