Which of the following is not an ore of magnesium?

Correct option is A

Magnesium is the eighth-most-abundant element in the Earth's crust by mass and tied in seventh place with iron in molarity. It is found in large deposits of magnesite, dolomite, and other minerals, and in mineral waters, where magnesium ion is soluble. Although magnesium is found in more than 60 minerals, only dolomite, magnesite, brucite, carnallite, talc, and olivine are of commercial importance.
Magnesite is a mineral with the chemical formula Mg C O3 (magnesium carbonate). Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts. Magnesite occurs as veins in and an alteration product of ultramafic rocks, serpentinite and other magnesium rich rock types in both contact and regional metamorphic terrains. These magnesites are often cryptocrystalline and contain silica in the form of opal or chert.
Magnesite is also present within the regolith above ultramafic rocks as a secondary carbonate within soil and subsoil, where it is deposited as a consequence of dissolution of magnesium-bearing minerals by carbon dioxide in groundwaters.
Dolomite forms in hydrothermal veins or as a pore-filling mineral in carbonate rocks, and more rarely as an accessory component in igneous pegmatites or altered mafic igneous rocks. By far though, most dolomite occurs in altered sedimentary marine rocks called dolostones or in marbles formed from the metamorphism of dolostone. Because dolostones are composed primarily of the mineral dolomite, geologists once used the term ‘dolomite’ for both the mineral and the rock. The term is now only used for the mineral, since a dolostone may include other minerals besides dolomite.
Few dolostones are primary in origin. In other words, they did not originally form as dolostone, but instead formed from the alteration of limestone rock as magnesium-rich water moved through the limestone, altering its calcite and aragonite into dolomite. 
Additional Knowledge
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. Gypsum is the most common sulfate mineral. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk chalk. Gypsum also crystallizes as translucent crystals of selenite. It forms as an evaporite mineral and as a hydration product of anhydrite.