Which diagnostic trouble code (DTC) indicates a problem with the fuel mixture that is too rich on Bank 2?

The diagnostic trouble code P0175 specifically indicates that the fuel mixture is too rich on Bank 2 of the engine. This code is triggered when the engine's oxygen sensors detect an excessive amount of fuel in the combustion process, which can lead to increased emissions and inefficient engine performance.

When examining other possible codes, P0128 refers to an engine coolant temperature problem, indicating that the engine is not reaching the required temperature for proper operation, rather than an issue with the fuel mixture. P0455 represents a large leak detected in the evaporative emissions control system, which is related to fuel vapor rather than the actual fuel mixture in the combustion chambers. Lastly, P2196 indicates a problem with the oxygen sensor signal that points to a rich condition in general but does not specify Bank 2.

Thus, P0175 is the most accurate code for identifying a rich fuel mixture specifically on Bank 2, making it the correct choice in this context.