Which of the phrases given in the options should replace the word/phrase that is underlined in the sentence to make it grammatically correct? If the sentence is correct as it is given and no correction is required, select ‘No correction required’ as the answer.

The 115km-long Panjshir valley, north of Kabul, is been flanked by snow-capped mountains up to 4000m high, with the only passable exits at the south and north of the valley.