720 thousand square meters should be 72 hectares. The answer should be (a)
At the same time, according to the information on the official website of the Forbidden City:
Palace buildings in the Forbidden City are the largest and most complete ancient buildings in China, with a total area of over 720,000 square meters. It is said that there are 9,999 semi-palaces, which are called "the sea of palaces" and are magnificent. Whether it is plane layout, three-dimensional effect or grand form, it is an unparalleled masterpiece.