B, sodium carbonate can react with dilute hydrochloric acid to generate gas, making test tube B bubble, but the generated carbon dioxide presses hydrochloric acid into test tube B, and the potassium chloride generated by the reaction of hydrochloric acid and potassium hydroxide is a water-soluble substance, which will not make test tube B turbid, so B is wrong;
C, iron can react with dilute hydrochloric acid to generate hydrogen, which makes tube B bubble. The generated hydrogen increases the pressure in A, dilute hydrochloric acid is pressed into test tube B, and the silver chloride generated by the reaction of hydrochloric acid and silver nitrate is insoluble in water and precipitated by acid, which makes the solution turbid, so C is correct;
D, sodium chloride and hydrochloric acid do not react, do not produce gas, and there will be no bubbles in test tube B, so D is wrong.
So choose C.