Step 1: Concept The Kroll process is the primary industrial method for producing metallic titanium.
Step 2: Meaning Titanium dioxide cannot be efficiently reduced by carbon or hydrogen; therefore, it must first be converted into a halide.
Step 3: Analysis In the Kroll process, titanium tetrachloride ($TiCl_{4}$) is reacted with liquid magnesium in an inert atmosphere to produce titanium sponge.
Step 4: Conclusion Thus, $TiCl_{4}$ is the specific compound that is reduced by magnesium to produce titanium.
Final Answer: (B)