Answer:
Exothermic reaction
Step-by-step explanation:
When heat is continuously supplied to a system and a spike in temperature is observed at any instantaneous point, it means that the bonds between atoms or molecules are being made. When bonds are made their is a sudden release of energy. Consider atoms that were initially moving fast with kinetic energy and they are forced to make bonds with each other which makes them loose their momentum, for momentum to be conserved that extra energy is released as heat, hence increasing the the internal energy of the system.