Final answer:
The correct sentence is option B: Walking on the treadmill is for the old; running, the young.
Step-by-step explanation:
The correct sentence is option B:
Walking on the treadmill is for the old; running, the young.
This sentence is punctuated correctly because it uses a semicolon to separate the two independent clauses ('Walking on the treadmill is for the old' and 'running, the young'). It also properly uses a comma to offset the nonessential phrase 'the young'.
Learn more about Punctuation in English sentences