Mount Everest is the highest mountain in the world. It is part of the Himalayan mountain range and is located on the border between Nepal and the Tibet region of China.

Mount Everest is about 8,848 meters (29,029 feet) tall. That is almost 9 kilometers high! Because it is the tallest mountain, many climbers from around the world try to reach its peak every year. Climbing Everest is very difficult and dangerous because of the freezing weather, low oxygen, and steep slopes.

The local name for Everest in Nepal is "Sagarmatha," and in Tibet, it is called "Chomolungma." The mountain was named "Everest" after Sir George Everest, a British surveyor general of India in the 1800s.

While K2 and Nanga Parbat are also very high mountains and part of the Karakoram range in Pakistan, they are not as high as Everest. K2 is the second-highest mountain in the world. Siachen is not a mountain, but a glacier, and it is famous for being a very cold and high-altitude military area.

Mount Everest was first successfully climbed in 1953 by Sir Edmund Hillary from New Zealand and Tenzing Norgay, a Sherpa from Nepal. Their achievement is still remembered as one of the greatest in history.

So, the correct answer is Everest, the tallest mountain in the world.