Reagan's biggest challenge during his first term as president was
iran hostage crisis
white house scandal
a failing economy
striking steel workers

The best and most correct answer among the choices provided by your question is the first choice or letter A which is "Iran hostage crisis."

The Iran hostage crisis was a diplomatic crisis between Iran and theUnited States. 52 American diplomats and citizens were held hostage for 444 days (November 4, 1979, to January 20, 1981) after a group of Iranian students belonging to the Muslim Student Followers of the Imam's Line, who supported the Iranian Revolution, took over the U.S. Embassy in Tehran. 

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The exchange of foods between the new and old worlds