Balanced stone is dental stone:: (A) Which undergoes rapid expansion, (B) In which, accelerators or retarders have been added according to need, (C) In which, amount of water of hydration is controlled, (D) In which, the crystals are all of uniform size
Answer is B. A stone with a setting time established by the addition of proper quantities of both accelerator and retarder is called ‘balanced stone’. 
Typical accelerators are potassium sulfate and potassium sodium tartrate (Rochelle Salts).
Typical retarders are sodium citrate and sodium tetraborate decahydrate (Borax).
Manappalil pg-313