Final answer:
Presidents often negotiate executive agreements over treaties as they do not require congressional approval. Example 'c' from the provided list is an executive agreement, and executive agreements can be less enduring than treaties.
Step-by-step explanation:
It is true that presidents often prefer to negotiate executive agreements rather than treaties because they do not require congressional approval. Executive agreements allow a president to formalize international relations without the constitutional requirement for the Senate to approve formal treaties, which require a two-thirds vote in the Senate to be ratified. This is a key reason why over 90 percent of international agreements the United States enters into are executive agreements rather than treaties.
An example of an executive agreement is option 'c': The president signs legally binding nuclear arms terms with Iran without seeking congressional approval. Additionally, a sole executive agreement is not necessarily likely to be in effect longer than a treaty, since it can be reversed by the next president while treaties are much more difficult to undo.