Option d is the correct answer. The first Sultan of Delhi to introduce the 'Haqq-i-Sharb' or irrigation tax was Firuz Tughluq. This tax, equivalent to one-tenth of farmers' crop yields, was utilized to fund the construction and maintenance of water distribution infrastructure, ensuring efficient irrigation facilities in the region under his rule. The haqq-i-sharbat was a progressive tax, meaning that the rate of tax increased with the amount of water used. This was to ensure that the wealthy farmers paid a higher proportion of the tax than the poor farmers.