In the form \(y'=f(t,y)\), the function \(f\) is exactly the right-hand side of the equation, which here is \(t+y\). The other options change a sign or an operator.
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.