Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is punctuation between a supplementary phrase and a main clause. This choice correctly uses a comma to mark the boundary between the introductory supplementary phrase (“By linking the Hudson River to Lake Erie”), which identifies how the canal reduced transport time, and the main clause (“the canal reduced transport time to nine days and cut costs to six dollars per ton”).
Why the other choices are wrong
Choice A
Choice A is incorrect because a semicolon can’t be used in this way to mark the boundary between a supplementary phrase (“By…Erie”) and the main clause (“the canal...ton”).
Choice B
Choice B is incorrect because an open parenthesis can’t be used in this way to mark the boundary between a supplementary phrase (“By…Erie”) and the main clause (“the canal...ton”).
Choice D
Choice D is incorrect because a colon can’t be used in this way to mark the boundary between an introductory supplementary phrase (“By…Erie”) and the main clause (“the canal...ton”).