The slope is evaluated at \(t_n\) and multiplied by \(h\), so the correct formula is \(y_n+h\,2t_n\). The other options use the endpoint of the interval or drop the dependence on \(t_n\).
Euler's method approximates the solution of an initial value problem \(y'=f(t,y)\), \(y(t_0)=y_0\), advancing in steps of size \(h\) using the slope at the current point.