How would you expect the appearance of a rock high in iron and magnesium to differ from a rock with very little iron and magnesium?

As explained below.

Explanation:

The rocks that have a higher content of magnesium and iron content are found in igneous rocks composition that is mafic and felsic in nature have a higher concentration of silica content also and are highly intrusive n nature are highly explosive and have a more acidic base. These rocks like the magmas of 750 to 950 degrees temperature are dark in color and higher pressure and others that don't have this high content are characterized as sedimentary or metamorphic in nature.  These have less density and light color with layered structures present in them.

And a) oxygen i believe

The answer is d.potassium