While A is certainly tempting here, we have to be very careful not to overextend our logic and draw conclusions that are beyond what the data can tell us. In this case, we can’t definitively conclude that Protein F has no quaternary structure because it is possible that Protein F is made up of multiple copies of the same subunit (a homodimer, for example). If broken down into its subunits, there would still only be a single band because they would be exact replicas of one another.If it was a native-PAGE i could see why it would be none of the above. However, this is SDS-page and the application of this type of detergent denatures protein molecules to mask their native charge so that only molecular size may be observed. Therefore in this situation, I chose A because quaternary structure is a type of structure where the protein has more than 1 type of subunit. There was only 1 dark clear band present in the results therefore I chose A.
Keep in mind that quaternary structure just refers to a protein with multiple subunits. It doesn't necessarily mean that the protein will be made up of distinct subunits, though.
I hope this helps!