ANS:D - Rain of molten lead.

In the Imperial Smelting Process (ISP) for extracting zinc, the term "rain of molten lead" refers to the technique used to quench the zinc vapor produced during smelting. As zinc vapor rises from the smelting furnace, it needs to be rapidly cooled to condense it back into a liquid state for collection. In the ISP, this cooling is achieved by allowing molten lead to cascade or fall in a manner resembling rain onto the rising zinc vapor. The molten lead absorbs heat from the zinc vapor, causing it to condense and form liquid zinc. This process of quenching with molten lead ensures efficient cooling of the zinc vapor and facilitates its collection for further processing. The molten lead does not react with the zinc vapor but serves as a medium for rapid heat transfer, allowing the zinc vapor to condense effectively.