If the value of H is positive, it means you have to add that much heat to complete the reaction. If H is negative, it means that much heat is released during the chemical process. Because it is -73 kJ, 73 kJ of heat are released in the reaction.
Answer: 100C of heat is needed.
Explanation: That is the heating point of water
I would say the last one because unlike beta and alpha rays they will have no effect on the mass or atomic number because they are just composed of high energy radiation