Answer:
D (Decomposition)
Step-by-step explanation:
This reaction is a decomposition reaction because one substance (NaNO3) is being separated into two substances (NaNO2 and O2). Alternatively, you could use the process of elimination. The reaction can't be single/double replacement because there's only one reactant, it can't be synthesis because substances are being broken down, not formed, and it can't be combustion because O2 is a product, not a reactant, and CO2 and H2O aren't being produced, as is common with combustion reactions.