What is an effective way to slow down or stop this reaction in aqueous solution?
naoh(aq)+kci(aq) -> naci (aq) +koh (aq)
increase the pressure of the reaction mixture
increase the surface area of the reactants
add a catalyst to the reaction mixture
add a lot of water to the reaction mixture

Add a lot of water to the reaction mixture.

Explanation:

The rate of the reaction is directly proportional to the concentration of the reactants.

So, adding a lot of water to the reaction mixture will decrease the concentration of the reactants. Thus, it slows down or stop the reaction.

So, the right choice is:

add a catalyst to the reaction mixture

it is important to conserve water because there is very limited amount of freshwater here on earth and if we are not smart about how we use it we will only be runnning out of our supply faster than neccessarry.