The Sunset Law mandated the auctioning of a zamindari if payment was not received by sunset on the specified date. This law stipulated that failure to pay land revenue by the end of the day would lead to forfeiture of the land. Its purpose was to enforce prompt tax payments from zamindars, thereby ensuring the timely collection of revenue vital for the colonial administration. Unlike the other options provided – the Regulating Act, Pitt’s India Act, and Martial Law – none of these specifically addressed the auctioning of zamindaris for non-payment by sunset. The Regulating Act focused on the administration and regulation of British territories in India, Pitt's India Act sought to improve the governance structure of India under British rule, and Martial Law involved the direct military control of normal civilian governmental functions.
Therefore, Sunset Law is the accurate designation.