Sentence Correction (SC) | Re: Hadrian's Wall in England is one of the various border defences built

I have a doubt here regarding the verb ing modifier lessening.

Is it modifying, 'who', which would refer to King Hadrian(which makes sense) or 'frontiers'(which doesn't make sense)

I understand the modifier with choice D wrt the Roman empire which had reached....However, for the doubt above i choseE