In choice (A), the pronoun "its" has no clear reference; there is no logical noun for it to refer to. For example, the balloon did not do the "leading." So we eliminate (A).
In choice (B), we have an incorrect modifier. The phrase before the comma ("In maintaining...was careless") grammatically modifies whatever comes after the comma, which as written, is "this carelessness," and would be nonsensical. The operator was careless, not carelessness itself. So we eliminate choice (B).
In choice (C), the pronoun "it" has no clear reference; there is no logical singular noun for it to refer to, so we eliminate (C).
Choice (D) is clear and correct. The subject "carelessness" matches the verb "led" in number (singular) and the tense is correct; since we are describing a past event, we use the past tense.
In choice (E), we have a run-on sentence, because there is no conjunction connecting the independent clause "it was maintained carelessly by the operator" with the rest of the sentence.
The correct answer is (D).