The five general chemical reactions are: Synthesis, Decomposition, Single displacement, Double displacement, and combustion.
25 - in this question we have some examples of the previous reactions
A) We have two ionic compounds undergoing a reaction in water, they will probably form two new compounds, therefore this is a classic Double displacement reaction
B) A single compound in the 5 general chemical reactions will be more likely to be placed in the Decomposition reaction spot, since in a reaction this single compound will end up forming 2 elements
C) Now we have the opposite from letter B, 2 elements forming a compound, this is a Synthesis reaction
D) Reactions involving oxygen as reactant are usually a Combustion reaction, and the products will be Carbon dioxide and water