The comity clause, found in Article IV, Section 1 of the U.S. Constitution, requires states to acknowledge the public acts, records, and judicial proceedings of other states. However, there are exceptions to this principle.
Among the options you've provided, b. charging more money for out-of-states college tuition is generally considered an exception to the comity clause. This is because states may have the discretion to set different tuition rates for in-state and out-of-state residents, and this practice does not necessarily violate the principle of comity.
The other options, particularly c. free traveling between states and d. recognizing all marriages from all states, align with the notions of comity and are generally upheld as states must recognize each other's rights and actions. Option a, buying and selling property in different states, also does not violate the comity clause itself, as states regulate property transactions.