Red Fort, also known as Lal Qila, is one of the most iconic historical monuments in India. It is located in Delhi and was built during the 17th century by the Mughal emperor Shah Jahan. The fort is made up of red sandstone, which is the correct answer.

1. Red Sandstone:
The Red Fort is primarily constructed using red sandstone, which gives it its distinctive red color. Red sandstone is a type of sedimentary rock that is composed of sand-sized grains of mineral, rock, or organic material. It is predominantly made up of quartz grains cemented together by iron oxide minerals, which gives it the reddish hue. Red sandstone is a popular building material in India and has been used in various architectural structures, including forts, palaces, and temples.

2. Black Sandstone:
Black sandstone is not used in the construction of the Red Fort. While black sandstone does exist and is found in different parts of India, it is not the material used in building the fort.

3. Red Marble and Black Marble:
Marble is a metamorphic rock that is commonly used in construction due to its durability and aesthetic appeal. However, the Red Fort is not made up of marble. Marble is primarily composed of calcium carbonate, and its colors range from white to various shades of gray, green, and even red. The use of marble in the construction of the Red Fort is limited to decorative elements such as intricate carvings and inlays.

In conclusion, the correct answer is option 'A' - Red sandstone. The Red Fort is predominantly made up of red sandstone, which gives it its iconic red color and is a testament to the architectural brilliance of the Mughal Empire.