Answer:
B. Mike joined the Army, and he became more disciplined during the training.
Step-by-step explanation:
Option B is the correct way to rewrite the run-on sentence because the two main independent clauses "Mike joined the Army" and "he became more disciplined during training" are joined by a connecting word "and" and a punctuation mark(comma). Both clauses are related thoughts. In other options the clauses are not joined correctly.