Does A nuclear reaction occurs when there is a change to the nucleus of an unstable atom?

Combustion is the act or process of burning. therefore, i believe the answer is  a: burning wood. 

Idon’t know what the answer is i wish i could

Answerδh > 0explanationc₆h₆  (s)  → c₆h₆ (l)if you are changing it from a solid to a liquid, you are pretty much melting the solid. this requires heat energy. therefore, the reaction is endothermic (takes in energy); the system enthalpy increases as the reaction proceeds: δh > 0.

Ithink the answer is d