Final answer:
Jonathan Swift's 'A Modest Proposal' uses sarcasm in option (a) to criticize British exploitation of Irish poverty by suggesting the selling of babies as a source of income.
Step-by-step explanation:
One instance of sarcasm in Jonathan Swift's A Modest Proposal is option (a): Swift's proposal to sell babies as a source of income, which criticizes the British exploitation of Irish poverty. Swift deliberately uses this shocking suggestion to satirize and lambast the cold, inhumane attitudes of the British towards the plight of the impoverished Irish people. The entire essay is a work of satire, meaning that it uses irony, sarcasm, and other comedic elements to criticize social issues and human follies.