Final answer:
The federal authorities finally put Al Capone behind bars on tax evasion charges, after a strategic focus on his businesses' failure to pay taxes. This led to his conviction in 1931 and an eleven-year prison sentence. Option a) is correct.
Step-by-step explanation:
The federal authorities wanted to jail Al Capone for his illegal activities during Prohibition. Despite being involved in various criminal operations such as bootlegging, gambling, and even murder, Capone evaded conviction for these crimes. Instead, the federal authorities turned their attention to his financial operations. The FBI focused on the fact that Capone's businesses, which were fronts for laundering his criminal profits, had not paid their taxes. This method proved effective, and in 1931 Capone was convicted of tax evasion.
Capone's trial for tax fraud resulted in an eleven-year prison sentence, which he served, in part, at the infamous Alcatraz prison. Despite his criminal empire and the violence associated with it, it was the strategic pursuit of Capone on charges of tax evasion that ultimately led to his incarceration.
Thus, the correct answer to the question of what the federal authorities did to finally put Al Capone behind bars is: a. Tax evasion charges.