The correct answer is option 'C', Rub al Khali Desert.

Rub al Khali, also known as the Empty Quarter, is considered the world's largest sand sea. It is located in the Arabian Peninsula and covers a significant portion of the southern part of Saudi Arabia, as well as parts of Oman, Yemen, and the United Arab Emirates. The desert spans an area of approximately 650,000 square kilometers, making it larger than the Gobi Desert (option A), Sahara Desert (option B), and Thar Desert (option D).

Here are some key details about Rub al Khali Desert:

1. Location:
- Rub al Khali Desert is primarily located in Saudi Arabia, covering a vast area of the Arabian Peninsula.
- It extends into neighboring countries, including Oman, Yemen, and the United Arab Emirates.

2. Size:
- Rub al Khali Desert is the largest continuous sand desert in the world.
- It spans an area of approximately 650,000 square kilometers, making it larger than any other sand desert on the planet.
- The desert stretches across multiple countries, covering a significant portion of the Arabian Peninsula.

3. Landscape and Features:
- The desert is characterized by massive sand dunes, some reaching heights of over 250 meters.
- It is a challenging and inhospitable environment, with extreme temperatures and limited water sources.
- The sand in Rub al Khali is predominantly reddish-orange, creating a stunning visual landscape.
- The desert also contains salt flats, gravel plains, and occasional oases.

4. Biodiversity:
- While the desert may appear barren, it supports some unique and adapted plant and animal species.
- Vegetation includes drought-resistant shrubs, grasses, and succulents.
- Desert-adapted wildlife, such as Arabian oryx, gazelles, and reptiles, can be found in some areas.

In conclusion, Rub al Khali Desert, also known as the Empty Quarter, is considered the world's largest sand sea. It spans an extensive area across Saudi Arabia, Oman, Yemen, and the United Arab Emirates, encompassing approximately 650,000 square kilometers of sand dunes, salt flats, and gravel plains.