Mean temperature of heat addition is ___________ due to regeneration

The correct option is increases.

Regeneration in a thermodynamic cycle (such as the Rankine or Brayton cycle) involves recovering waste heat from the exhaust working fluid and using it to preheat the colder working fluid before it enters the heat addition device (like a boiler or combustor).

Without regeneration, the working fluid enters the heat addition stage at a relatively low temperature. Consequently, heat must be added externally starting from this low temperature up to the maximum cycle temperature.
By implementing regeneration, the entry temperature of the working fluid to the combustor or boiler is raised. As a result, the external heat addition process begins at a higher temperature.

The mean temperature of heat addition (T_m) is a key thermodynamic parameter defined by the relation:

$T_m=\frac{Q_{\mathrm{in}}}{\Delta S}$

Since the lower-temperature region of external heat addition is eliminated by preheating the fluid internally, the average temperature at which external heat is supplied to the cycle is elevated. Therefore, the mean temperature of heat addition increases, which directly enhances the thermal efficiency of the cycle.