Q. Which of the following is the most important property of the molding sand in cores?

Explanation by: Mr. Dubey

 after the molten metal in the mould gets solidified, the sand mould must be collapsible so that free contraction of the metal occurs and this would naturally avoid the tearing or cracking of the contracting metal. in absence of collapsibility property, the contraction of the metal is hindered by the mold and thus results in tears and cracks in the casting. this property is highly required in cores.