A) The elections
are seen as a direct contest between sitting US President and Republican nominee Donald Trump and Democratic nominee Joe Biden, who
was vice-president during the Barack Obama presidency.
B) The elections are seen as a direct contest between sitting US President and Republican nominee Donald Trump and Democratic nominee Joe Biden, who
were vice-president during the Barack Obama presidency. (INCORRECT)
C) The elections are seen as a direct contest between sitting US President and Republican nominee Donald Trump and Democratic nominee Joe Biden,
during Barack Obama’s presidency, was Vice-President. (During Barack Obama's presidency looks like an irrelevant modifier, which is indeed important) (INCORRECT)
D) The elections
have been seen as a direct contest between sitting US President and Republican nominee Donald Trump and Democratic nominee Joe Biden, who
was vice-president during the Barack Obama presidency.
E) The election is seen as a direct contest between the sitting US President and Republican nominee Donald Trump and Democratic nominee Joe Biden, who
were the vice-president during the Barack Obama presidency.(INCORRECT)
Joe Biden is a single noun, using were is incorrect.
Even though are seen and have been seen both are present continuous, we are particularly talking about the case of two candidates Donald Trump and Joe Biden whose elections are recent action as depicted by are seen As compared to the present perfect continuous 'have been' which states action is been continued from more earlier than the recent past, that's why 'Are seen is preferable'.
Hence Option A is best option among all