What were the chief priorities of american diplomacy at the end of the nineteenth century?

The protection of the Monroe Doctrine and Open Door Policy from German and Japanese expansion into the Pacific and Asia

Explanation

The Senate that was mainly republican wanted to protect the Monroe Doctrine and the “open door” policy and refused to  interfere more in the European affairs.    

explanation:

there are two statements that are incorrect.

1. the marshall plan was a lend lease program. it was an economic plan that europe redevelop itself after wwii.

2. einstein didn't really do anything to further the allied cause in building the bomb. he merely sent a letter to fdr telling him (in 1939) that if the united states entered the war, perhaps it would be wise to build the bomb before germany did. i would say this answer is incorrect, but when america entered the war 2 years later, fdr thought it might be a good idea to take einstein's advice.

the 4th choice is definitely so wrong that is likely the single answer of this question

the completion of the erie canal spurred the first great westward movement of american settlers, gave access to the rich land and resources west of the appalachians and made new york the preeminent commercial city in the united states.