a Today, despite widely available technology such as high-resolution scanners and printers, counterfeiting is more difficult than it was at the time of the Civil War, when it was estimated that one-third of all currency in circulation was counterfeit.
Remove "despite widely available technology such as high-resolution scanners and printers" and you are left with Today counterfeiting is more difficult than it was at the time of the Civil War, when it was estimated that one-third of all currency in circulation was counterfeit.
Comparing today's situation with previous situation "it was".
b Despite widely available technology such as high-resolution scanners and printers today, counterfeiting is more difficult than it was at the time of the Civil War, when it was estimated that one-third of all currency in circulation was counterfeit.
Not comparing present time with past so incorrect.
c Despite widely available technology such as high-resolution scanners and printers, counterfeiting is more difficult than at the time of the Civil War, when it was estimated that one-third of all currency in circulation was counterfeit.
Not comparing present time with past so incorrect.
d Today, despite widely available technology such as high-resolution scanners and printers, counterfeiting is more difficult than when it was estimated that one-third of all currency in circulation was counterfeit at the time of the Civil War.
Remove "despite widely available technology such as high-resolution scanners and printers" and you are left with Today counterfeiting is more difficult than when it was estimated that one-third of all currency in circulation was counterfeit at the time of the Civil War.
Compares Today's situation with previous time "when". In correct comparison.
e Today, because of widely available technology such as high-resolution scanners and printers, counterfeiting is more difficult than it was at the time of the Civil War, when it was estimated that one-third of all currency in circulation was counterfeit.
Indicates "because of widely available technology" counterfeiting is difficult.
I will go with A.