Which one of the following elements cannot be detected by "Lassaigne's test"?

Lassaigne's test, also known as the sodium fusion test, is a qualitative analytical technique used to detect nitrogen, sulfur, and halogens (chlorine, bromine, and iodine) in organic compounds. During the test, the organic compound is fused with metallic sodium to convert covalent elements into water-soluble sodium salts like NaCN, Na2S, and NaX. While chlorine, bromine, and iodine are detected by reacting the sodium extract with silver nitrate to form characteristic precipitates (AgCl, AgBr, and AgI), fluorine cannot be detected using this method. This is because silver fluoride (AgF) is highly soluble in water, unlike other silver halides, and therefore does not form a visible precipitate to confirm its presence. Consequently, fluorine is the element among the options that cannot be detected by Lassaigne's test.