Which one of the following is not an igneous rock ?

Rocks are classified into three main types based on their mode of formation: igneous, sedimentary, and metamorphic. Igneous rocks form from the cooling and solidification of molten rock (magma or lava).

• Granite: A common intrusive igneous rock formed from slow-cooling magma deep beneath the Earth's surface.
• Basalt: An extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava at the surface.
• Gabbro: A coarse-grained, intrusive igneous rock chemically equivalent to basalt.

Slate is the correct answer because it is a metamorphic rock, not an igneous one. It originates from the transformation of shale (a sedimentary rock) through low-grade regional metamorphism involving intense heat and pressure, which causes the clay minerals to realign into thin, flat layers.