Answer : The internal energy of the system within the cylinder increase during the change.
Explanation :
Endothermic reaction : It is a type of chemical reaction in which the energy is absorbed from the surrounding and given to the system.
Exothermic reaction : It is a type of chemical reaction in which the energy is released into the surrounding.
The formula of internal energy is:

The internal energy is temperature dependent.
As, temperature increases internal energy will be increases and vice-versa.
As per question, if the process is endothermic, that means the heat is supplied to the system. Hence the internal energy of the system within the cylinder increases as temperature is increasing.
Hence, the internal energy of the system within the cylinder increase during the change.