The first computerized spreadsheet, Dan Bricklin’s VisiCalc , improved financial recordkeeping not only by

Choice C is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period is used correctly to mark the boundary between the first sentence (“The...adjustments”) and the second sentence (“Prior...days”). Because the adverbial phrase beginning with “prior” indies when changing a spreadsheet required redoing the sheet by hand, that phrase belongs with the second sentence. 

Choice A is incorrect because it results in a run-on sentence. Two sentences are fused without punctuation and/or a conjunction. Choice B is incorrect because it results in a comma splice. A comma can’t be used in this way to mark the boundary between sentences. Choice D is incorrect. Without a comma preceding it, the conjunction “and” can’t be used in this way to join the sentences.