f:=2 t/(1+t^m);
m=4;
abb1=Plot[f,{t,0,2},ImageSize->{400,300},DisplayFunction->Identity];
m=5;
abb2=Plot[f,{t,0,2},ImageSize->{400,300},DisplayFunction->Identity];
m=6.4;
abb3=Plot[f,{t,0,2},ImageSize->{400,300},DisplayFunction->Identity];
m=8;
abb4=Plot[f,{t,0,2},ImageSize->{400,300},DisplayFunction->Identity];
bild=Show[abb1,abb2,abb3,abb4,AxesLabel->{"c","f(c)"},
DisplayFunction->$DisplayFunction]