Are the Following Functions Linear or Nonlinear? Explain ${}^{-} \frac { 1 } { 4 } x + 3 y = 0$

Are the following functions linear or nonlinear? Explain.
A) ${}^{-} \frac { 1 } { 4 } x + 3 y = 0$
B) y = x(x-2)
C) y = |-2|x
D) A = $\pi$ r²