Which of the following reactions represents $\Delta H_f^0$ ?

$\Delta H_f^0$ denotes the standard enthalpy of formation. It states that during the formation of 1 mole of substance when its constituent elements are in standard states, the enthalpy change is represented by the standard enthalpy of formation.

Among the given reaction equation, the reaction that follows this standard enthalpy of formation is, $\frac{1}{2}{H}_{2\left(g\right)}+\frac{1}{2}{F}_{2\left(g\right)}\to H{F}_{\left(g\right)}$.

Here 1 mole of substance is formed and all the constituent elements are in the standard states.

Therefore, option $\left(B\right)$ is correct.