Answer:
The most important security measure of the 100 dollar bill in its 2013 series is the EURion Constellation, because it provides security against counterfeiting of banknotes through digital printing that cannot be reproduced by conventional printers, not even if these are of high quality or advanced technology. In this way, only banknotes printed in the Federal Reserve can reproduce this pattern, with which its legitimacy is immediately verified.
Other protections, such as watermarks or the plastic security band, also help to increase the security of the ticket, but none achieves this in the same proportion as the EURion Constellation.