The question asks to identify the ultramafic rock from the given options. Ultramafic rocks are igneous rocks composed primarily of silicate minerals rich in magnesium (Mg) and iron (Fe), with very low silica (SiO₂) content. They typically contain minerals like olivine and pyroxene.

Rock Type Identification

• Granite: This is a felsic intrusive igneous rock, characterized by high silica content and abundant quartz and feldspar. It is not ultramafic.
• Gabbro: This is a mafic intrusive igneous rock. While it contains mafic minerals like pyroxene and plagioclase feldspar, it has a higher silica content than ultramafic rocks.
• Dunite: This is an ultramafic intrusive igneous rock. It is almost entirely composed (>90%) of olivine, a magnesium iron silicate mineral. This fits the definition of an ultramafic rock.
• Basalt: This is a mafic extrusive igneous rock, chemically equivalent to gabbro but with a finer grain size due to rapid cooling. It is not ultramafic.

Conclusion on Ultramafic Rocks

Based on the mineral composition, Dunite is the only ultramafic rock among the choices provided.