Select the correct statement for 50% reaction stage in a steam turbine.

To understand the relations in a 50% reaction stage steam turbine, let us analyze the velocity triangles and thermodynamic definitions of a reaction stage.

The degree of reaction ($R$) is defined as the ratio of the static enthalpy drop in the rotor to the total static enthalpy drop in the stage:
$R=\frac{\mathrm{Static\; enthalpy\; drop\; in\; rotor}}{\mathrm{Total\; static\; enthalpy\; drop\; in\; stage}}$

For a 50% reaction stage ($R=0.5$), the static enthalpy drop (and thus the pressure drop) is equally divided between the stator (fixed blades) and the rotor (moving blades). This symmetrical distribution of enthalpy drop leads to highly symmetric velocity triangles.

Let the velocity parameters be defined as follows:
- ${\alpha }_1$ = Absolute inlet flow angle (exit angle from stator)
- ${\beta }_1$ = Relative inlet flow angle (inlet angle of rotor blade)
- ${\alpha }_2$ = Absolute exit flow angle (exit angle from rotor)
- ${\beta }_2$ = Relative exit flow angle (exit angle of rotor blade)

For a 50% reaction stage, the moving blades (rotor) and the fixed blades (stator) have the same shape, which yields the following symmetric relations between the flow angles:
${\alpha }_1={\beta }_2$
and
${\beta }_1={\alpha }_2$

The relation ${\alpha }_2={\beta }_1$ states that the absolute exit flow angle (${\alpha }_2$) is equal to the inlet angle of the rotor blade (${\beta }_1$). This matches the correct option.