Answer: B (hydrogen). Polyunsaturated vegetable oils contain carbon–carbon double bonds. These C=C bonds put kinks in the molecules so they cannot pack closely, which is why the oils are liquid at room temperature.
To make solid margarine, hydrogen is added across the double bonds in a process called hydrogenation, using a nickel catalyst at about 180 °C. The C=C bonds become saturated C–C single bonds, the molecules straighten and pack together more tightly, and the melting point rises so the oil sets to a solid.
Bromine (A) also adds across C=C bonds, but it gives a dibromo compound, not margarine. Oxygen (C) would oxidise the oil and make it go rancid rather than solidify it. Water (D) does not add across isolated C=C bonds under these conditions. So the substance added is hydrogen, option B.
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