This question was addressed to me in an online interview.
The doubt is from Core Making Machines in section Foundry Mechanizations of Casting-Forming-Welding I
Explanation: The cores are compact mass of core sand which is a special kind of sand. And cores are always prepared separately and then placed in mould cavity at a required location with proper alignment and accuracy. Core does not allow the liquid metal to occupy space for solidification in that part and hence help to produce hollowness in the casting component.