What seemed as remarkable as the medieval invention of playing cards are the thousands upon thousands of unique games that are shared by a worldwide audience.
we can easily eliminate C D and E. C uses the past perfect (not necessary and badly used), D has an awkward idiom (nonetheless remarkable than) and E uses a wrong idiom:
"as remawkable as the thousands" would be correct, the
are makes it wrong.
C The concept that
had been as remarkable as the medieval invention of playing cards
D The medieval invention of playing cards has been
nonetheless remarkable thanE Medieval invention of playing cards was just as
remarkable as (are) A and B remain. In A
seemed in incomplete, a "to be" is necessary : what seemed to be is the correct form. B wins
A What
seemed as remarkable as the medieval invention of playing cards
B No less remarkable than the medieval invention of playing cards
Hope it's clear