pi=Pi//N;
f=a/(2 c Sqrt[pi t]) Exp[-x^2/(4 c^2 t)];
a=1;c=1;
bild=Plot3D[f,{x,-2,2},{t,0.05,0.5},PlotPoints->30,PlotRange->All,
AxesLabel->{"x","t","f"},ImageSize->{400,300}];