Answer is : (a) volatile stable compound
Ultra pure metals, are prepared by the van-Arkel method. Arkel method is used to purify oxide of titanium metal. Ti is heated with iodine to about 500 K to form volatile compound. The \(Til_4\) is reheated at 1700 K, when it decomposes to give pure Ti.
\(Ti+2l_2\overset{500 k}{\rightarrow} \underset{\text{Volatile stable compound}}{Til_4}\) \(\overset{1700 k}{\rightarrow} \underset{\text{Pure metal}}{Ti}+2l_2\)
