To ensure that the reaction between two 1 liter gas samples in a sealed flask proceeds very slowly, the best approach would be to keep the flask.
Increasing the concentration of each gas (option a) would actually speed up the reaction, as higher concentrations typically lead to more frequent collisions between gas molecules. Similarly, using a larger flask (option c) could also lead to a faster reaction due to increased volume allowing for more molecule interactions.
By simply keeping the flask as it is, you minimize unnecessary interactions that could accelerate the reaction. This way, you can create conditions where the reaction proceeds at a more controlled and slower rate.
Therefore, the most appropriate choice is b) keep the flask.