Some atoms become unstable because electrons and protons are off-balance neutrons and protons are off-balance neutrons and electrons are off-balance neutrons become attracted to other elements

Some atoms become unstable because NEUTRONS AND PROTONS ARE OFF BALANCE.
Neutrons and protons are the subatomic particles, which are found inside the nucleus. A nucleus is described as stable when its binding energy is enough to hold the nucleus together. When the binding energy is not enough to hold the nucleus together, the nucleus is described as unstable. In order to become stable the nucleus will lose protons and neutrons. 

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