Which of the following is not a factor that determines if a volcano erupts explosively or quietly?
a. the magma's composition
b. temperature of the magma
c. size of the volcanic cone
d. amount of dissolved gases in the magma

Option C, size of the volcanic cone

Explanation:

The explosion of vaolcano depends on -

a)magma composition – If magma is composed of some material which has low viscosity, the already low  viscosity further goes lower on heating of magma. Thus it becomes very easy for the fluid to flow. Felsic or rhyolitic components in magma has a high viscosity as compared to magma containing mafic or basaltic components.  

b)magma temperature – The higher the temperature, the lower  would be the viscosity of the material of which a volcano is composed of and the higher will be its fluidity.

c)the amount of dissolved gases that are contained in the magma- When high amount of gases are  dissolved in magma , the pressure of the magma also become high and  thus the volcanic eruptions are explosive.