\(A=x^2-2xy+y^2-z^2=\left(x-y\right)^2-z^2=\left(x-y+z\right)\left(x-y-z\right)\)
\(B=3x^2-8x+4=3x^2-6x-2x+4=3x\left(x-2\right)-2\left(x-2\right)=\left(x-2\right)\left(3x-2\right)\)
\(D=4x^4+81=\left(2x^2+9\right)^2-36x^2=\left(2x^2+6x+9\right)\left(2x^2-6x+9\right)\)
\(C=x^4+4^2=\left(x^2+4\right)^2-8x^2=\left(x^2+\sqrt{8}x+4\right)\left(x^2-\sqrt{8}x+4\right)\)