The answer is C. Eastern coast of Africa.
Zheng He's expedition reached the eastern coast of Africa on his fourth voyage, which took place from 1417 to 1419. He visited the ports of Mogadishu, Malindi, and Mombasa, and may have even reached as far south as Mozambique. This was the furthest extent that any Chinese expedition had ever reached at that time.
The other options are incorrect. Egypt was reached on Zheng He's second voyage, the southern tip of India was reached on his third voyage, and Australia was not discovered until the 17th century.