What seemed as remarkable as the medieval invention of

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 than
E 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