Answer:
Because.
Step-by-step explanation:
Transition words or phrases are those words or groups of words that act as the joining of two ideas or statements. They also act as the medium of transitioning from one idea to another, though they may not have the same idea. The ideas may b similar or even contrasted against each other.
In the sentence given-
By late summer 1793, the yellow fever epidemic had taken hold of Philadelphia. ______ Henry Knox feared catching the fever if he remained there, he fled to Manhattan Island.
The transition phrase/ word best suited to join these two sentences/ideas will be 'BECAUSE' for it rightly gives the cause of the first statement. The first sentence talks of the epidemic which led to the second sentence to happen, thus 'BECAUSE' provides the transitioning of the two ideas.
Thus, the final sentence will become
By late summer 1793, the yellow fever epidemic had taken hold of Philadelphia. Because Henry Knox feared catching the fever if he remained there, he fled to Manhattan Island.