Overview
Is the history of US foreign policy from 1776 to the present more a story of continuity or change? (or somewhere in the middle, context dependent, etc.)
There is not a specific right or wrong answer to this question - the important thing is that you formulate a clear thesis, structure your paper well, and follow the claim-evidence-reasoning format.
Questions/issues to consider
When making your argument, consider whether you intend to discuss continuity and change in the context of policy decisions, policy principles, general outlook, goals, or some combination of the above.
When arguing for continuity or change, make sure to separate what external forces were and what internal forces were. Is there some constant pattern or foundation which explains different reactions to different external events?