Final answer:
Diamonds are found in kimberlite pipes, which are rare ultramafic volcanic structures brought to the surface through eruptions, different from the felsic rock rhyolite.
Step-by-step explanation:
Diamonds are primarily found in a type of rare ultramafic volcanic pipe. The correct answer to the question is that diamonds are found in c) kimberlite pipes, not a) pyroclastite, b) rhyolites, or d) volcanite. Kimberlite is a type of igneous rock best known for containing diamonds. It forms deep within the Earth's mantle and is brought to the surface through volcanic eruptions. Unlike rhyolite, which is felsic and has an aphanitic texture due to the rapid cooling of high-viscosity lava, kimberlite is an ultramafic rock that originates much deeper in the Earth and is the primary source of diamonds.