Other politicians may have claims to seniority over their peers, based upon their accomplishments and their ability to lead campaigns, but Alexandria Ocasio-Cortez, who is often described as merely being a junior member of the House of Representatives, has an unparalleled seniority over all of them by virtue of her prowess in social media management.

The phrase 'merely being' is awkward as it introduces redundancy in the sentence. Apart from that, to 'be' a junior representative is often considered the right terminology. The sentence already uses “described as,” which means it refers to a state or identity, not an action that requires “being.” All of options A, B, and C fail to eliminate the term. Option D, 'as a mere junior member of the house of representatives' is correct.