The correct answer is A. He told us to meet him at 6 o'clock, and he promised to be on time.
Step-by-step explanation:
According to grammar, if a sentence contains two or more independent clauses these need to be separated either using a semicolon or a punctuation mark such as comma followed by a conjunction, in the cases this is omitted and the clauses are placed together a run-on sentence occurs which is a type of grammatical error.
In the sentence "He told us to meet him at 6 o'clock and he promised to be on time" the writer has used only one conjunction (and) to try to separate the clause, but this should be preceded by a comma or the conjunction should be omitted and instead a semicolon should be used. According to this, the correct revision for this is "He told us to meet him at 6 o'clock, and he promised to be on time" because this includes the comma before the conjunction that is essential to separate both clauses correctly.