Marble is classified as a metamorphic rock. Metamorphic rocks are formed when pre-existing rocks, such as sedimentary or igneous rocks, undergo extreme heat and pressure. In the case of marble, it is formed from limestone or dolomite, which are sedimentary rocks, through a process called metamorphism. During this process, the mineral composition and structure of the rock change, resulting in the formation of marble. Marble is known for its distinctive crystalline texture and varied colors, which are a result of the geological processes it undergoes. Therefore, the correct answer is option 3 - Metamorphic.