The best correction of the error is the sentence <em>Because the last train is leaving the station, it is definitely time to get on board.</em>
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Explanation:
A comma splice, also referred to as a comma fault, is a mistake made when a comma is used to connect two independent clauses without the use of a conjunction. The given sentence can be split into two independent clauses:
The last train is leaving the station.
It is time to get on board.
The best correction is the choice C) <em>Because the last train is leaving the station, it is definitely time to get on board</em>, because it is the only choice that has the same meaning as the original sentence. <em>Because</em> is a subordinating conjunction that shows a cause-and-effect relationship between a subordinate clause and the main clause. The subordinate (dependent) clause in the choice C is <em>Because the last train is leaving the station. </em>The train leaving is a cause of what is stated in the independent (main) clause: <em>it is definitely time to get on board. </em>Conjunctions used in other choices don't have this meaning.
