c,
`(x^2-xy-xz+yz)/(x^2+xy-xz-yz)`
`=(x(x-y)-z(x-y))/(x(x+y)-z(x+y))`
`=((x-z)(x-y))/((x-z)(x+y))`
`=(x-y)/(x+y)`
d,
`(x^3-x^2-x+1)/(x^3+1)`
`=(x^2(x-1)-(x-1))/((x+1)(x^2-x+1))`
`=((x^2-1)(x-1))/((x+1)(x^2-x+1))`
`=((x-1)(x+1)(x-1))/((x+1)(x^2-x+1))`
`=(x-1)^2/(x^2-x+1)`
`=(x^2-2x+1)/(x^2-x+1)`