Final answer:
Jefferson feared that the Kentucky Resolution might encourage secessionist movements in other states.
Step-by-step explanation:
Jefferson's fear about the Kentucky Resolution was letter b) He feared it might encourage secessionist movements in other states.
The Kentucky Resolution, drafted secretly by Jefferson and James Madison in response to the Alien and Sedition Acts of 1798, declared that states had the right to judge the constitutionality of Acts of Congress and to declare such Acts null if found to be unconstitutional.
Jefferson worried that this assertion of states' rights could inspire other states to consider secession, which would threaten the unity and stability of the young nation.