Option b is the correct answer. The iron column, located in the courtyard of Delhi's Quwwat-ul-Islam Mosque, is a retention of the Gupta emperor Chandragupta II, who is also known as Chandra. The column bears an inscription in Sanskrit that mentions his name and praises his achievements. The inscription also states that the column was originally erected on the hill of Vishnupada, which is identified as the Udayagiri Caves in Madhya Pradesh. The column was later moved to Delhi by Anangpal Tomar, a ruler of the Tomara dynasty, in the 11th century CE. The column is famous for its rust-resistant composition of iron and phosphorus, which has preserved it for over 1600 years.