Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of a colon within a sentence. In this choice, the colon is correctly used to introduce the following explanation of what the Patriot cause had grown into by 1774.
Why the other choices are wrong
Choice B
Choice B is incorrect because a semicolon can’t be used in this way to join the main clause ("what…extensive") and the supplementary element ("a network…Congress"). A semicolon is conventionally used to join two main clauses, whereas a colon is conventionally used to introduce an element that explains or amplifies the information in the preceding clause, making the colon the better choice in this context.
Choice C
Choice C is incorrect because it results in a comma splice. The addition of the pronoun and verb "it was" forms the start of a new main clause in the sentence, and a comma can’t be used in this way to mark the boundary between two main clauses.
Choice D
Choice D is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with "a network."