Final answer:
A political event is most likely to impact a defense contractor that relies on large government contracts for its success.
Step-by-step explanation:
Out of the given options, the external event that is most likely to impact a defense contractor that relies on large government contracts for its success is a political event.
Political events, such as changes in government policies, regulations, or budgets, can directly affect defense contractors. For example, if there is a shift in government priorities or a change in defense spending, it can impact the availability and size of government contracts for defense contractors. This can have a significant impact on the success and profitability of a defense contractor that relies heavily on government contracts.