The correct answer is "The new electric pencil sharpener does not sharpen well; I prefer the old manual one."
Explanation:
In grammar, to combine to sentence often implies to use a punctuation mark or transition to create a longer and more complex sentence, which varies depending on the type of sentences and the relation between both.
In the case presented, the two sentences "The new electric pencil sharpener does not work properly" and "I prefer the old manual one" are independent clauses, this means they can stand by themselves and express complete ideas. Due to these, according to grammar these sentences can only be linked using a semicolon (;) or a comma (,) and a conjunction, this means, from the options presented the only one that shows a correct way to combine these sentences si "The new electric pencil sharpener does not sharpen well; I prefer the old manual one".
