The correct response would depend on several factors, including the initial temperatures of substances A and B, their specific heat capacities, and mass. However, generally speaking, when two substances at different temperatures are mixed, they will tend to reach thermal equilibrium, which is a common principle in thermodynamics.
If substance A is warmer than substance B, heat will flow from substance A to substance B until they reach the same temperature. If we assume that substance A and substance B are mixed and they do not undergo any phase change or chemical reaction, the final equilibrium temperature (T_final) can be calculated using the formula for heat exchange:
\[ m_A \cdot c_A \cdot (T_{A} - T_{final}) = m_B \cdot c_B \cdot (T_{final} - T_{B}) \]
In general terms, it's incorrect to specify that they will both reach exactly 50°C or 25°C unless those are the initial temperatures of both substances and no heat loss to the environment occurs.
Therefore, the best general answer would imply that they will reach equilibrium at some temperature that is dependent on the specific conditions of the substances involved, rather than one of the specific statements you've provided.