The correct answer is option 2, the Eran inscription of Bhanu-gupta. The inscriptions mentioned in options 1, 3, and 4 do not refer to the sati custom.

The Eran inscription of Bhanu-gupta is the earliest known reference to the sati custom. Sati, also known as suttee, was a practice in ancient India where a widow would immolate herself on her husband`s funeral pyre. The inscription, dated to the 5th century CE, mentions the construction of a temple dedicated to a queen who performed sati. This inscription provides historical evidence of the existence of the sati custom during that time period.

Alert - correct answer should be option 3, the Aihole inscription of Pulkes-in II. The Aihole inscription of Pulkes-in II, dating back to the 7th century CE, is the earliest known reference to the sati custom in ancient Indian inscriptions.