The option which corrects the error is B. radio's. Given that there is only one radio and that it has knobs, it means that the knobs belong to it. In order to show such a possession, you need to use the apostrophe after the word, and then add the suffix -s if there is only one item. So, one radio becomes radio's. The other options do not make any sense, given that the possession should be placed on the word radio, not knobs or numbers.
