When carbon dioxide (CO2) reacts with water (H2O), it forms carbonic acid (H2CO3). So, the correct answer from the options provided is d) H2CO3.
To explain further, this reaction occurs when CO2 is dissolved in water, which can happen naturally, for instance, when carbonated beverages are made. The reaction can be represented by the following chemical equation:
CO2 + H2O ⇌ H2CO3
This is an important process in various environmental and biological contexts, such as regulating pH levels in blood and oceans.