Which type of reaction occurs without being started by some outside force?
combustion reaction
physical reaction
spontaneous reaction
activation energy reaction

Option (c) is the correct answer.

Explanation:

A spontaneous reaction is a reaction which occurs on its own without any intervention or outside force helping in initiation of the reaction.

A combustion reaction is the reaction in which a compound reacts with oxygen atom resulting in the formation of water and carbon dioxide gas.

A physical reaction is a reaction in which there occurs no change in chemical composition of a substance.

Activation energy is the reaction in which minimum amount of energy is required to carry out a reaction. More is the activation energy of a reaction slower will be the rate of reaction, therefore a catalyst is required to increase the rate of reaction.

Thus, we can conclude that spontaneous reaction is the type of reaction that  occurs without being started by some outside force.