Final answer:
Calhoun's statement comparing Southerners to serfs due to the economic dependency created by the tariff is ironic given the real, literal slavery present in the South, with Southerners being slaveholders themselves.
Step-by-step explanation:
When John C. Calhoun wrote that the tariff made "serfs of the Southerners," he was referring to the economic dependency created by the Tariff of 1828, likening Southerners to serfs because of their perceived subjugation under economically oppressive conditions. Specifically, option (a) Calhoun referred to economic dependency, likening Southerners to serfs is the correct interpretation of Calhoun's statement. The use of 'serfs' here suggests that Southerners were bound and heavily burdened by the tariff policies, much like medieval serfs were bound to the land and subject to the will of their lords.
The statement is ironic because while Calhoun and other Southerners complained of metaphorical serfdom due to tariffs that advantaged Northern manufacturers, the South's actual economy was built upon the enslavement of African Americans. Thus, while claiming to be in servitude, Southern slaveholders were themselves imposing far more severe and literal conditions of servitude upon millions of enslaved individuals, ignoring their own role in maintaining an institution that truly dehumanized and subjugated people.