The statement that is necessarily correct about the reaction is that both calcium carbonate and sodium carbonate are being produced
2NaCl + CaCO3 ⇌ Na2CO3 + CaCl2
The reaction above is a reversible reaction. A reversible reaction is a reaction in which the conversion of reactant to products and the conversion from product to reactant occurs simultaneously. This reaction is just like a back and forth reaction.
⇌ symbol represents a reversible reaction. Therefore, the statement that is necessarily correct about the reaction is that both calcium carbonate and sodium carbonate are being produced