How much of a gas stays dissolved in liquid metal depends directly on the gas pressure sitting above the melt, the lower that outside pressure, the less the metal can hold on to. Putting molten steel under vacuum drops the pressure above the bath sharply, so hydrogen and nitrogen that were dissolved inside come back out of solution and get pumped away. Oxides and slag are separate solid or liquid phases removed mechanically by skimming, and any carbon drop under vacuum is really a side effect of the carbon oxygen reaction rather than the main target. What vacuum treatment is really built to strip out is trapped gas, so the answer is dissolved gases, option (D).