**Explanation:**

The Taj Mahal, one of the most famous and beautiful monuments in the world, is made up of **marble**, which is a metamorphic rock. Here is a detailed explanation of why the correct answer is option 'D':

**Marble: A Metamorphic Rock**

1. **Definition of Marble:** Marble is a metamorphic rock that is formed from limestone or dolomite under intense heat and pressure. It undergoes a process called metamorphism, which causes the original rock to recrystallize and develop a new texture and composition.

2. **Formation of Marble:** Marble is formed when limestone or dolomite is subjected to high temperatures and pressures deep within the Earth's crust. This can occur during mountain-building processes or when the rocks are buried deep underground.

3. **Characteristics of Marble:** Marble is known for its beautiful appearance and is often used in sculptures, buildings, and monuments due to its aesthetic qualities. It has a smooth texture, a range of colors (white, gray, pink, green, etc.), and can be polished to a high shine.

4. **Taj Mahal's Construction Material:** The Taj Mahal, located in Agra, India, was built by the Mughal emperor Shah Jahan in the 17th century as a mausoleum for his beloved wife, Mumtaz Mahal. The monument is primarily constructed using white marble, which gives it its iconic and dazzling appearance.

5. **Importance of Marble in Taj Mahal:** The use of marble in the construction of the Taj Mahal was a deliberate choice by the Mughal architects and artisans. Marble was selected for its beauty, durability, and ability to be intricately carved and decorated. The white marble used in the Taj Mahal is highly prized for its purity and translucency, which allows the monument to glow in different shades depending on the time of day.

In conclusion, the Taj Mahal is made up of marble, which is a metamorphic rock. The use of marble in its construction adds to its grandeur and timeless beauty.

Marble is a metamorphic rock.