Let me add more clarification to the POE.
By the mid-seventeenth century, Amsterdam had built a new town hall so large that only St. Peter's in Rome, the Escorial in Spain, and the Palazza Ducale in Venicecould rival it for scale or magnificence.
(A) could rival it for - looks perfectly fine
(B) were the rivals for it in their - were denotes change of tense.
(C) were its rival as to - same as B
(D) could be its rivals in their - 'their' is ambiguous
(E) were rivaling its - same as B
Kudos
...
By the mid-seventeenth century, Amsterdam had built a new town hall so large that only St. Peter's in Rome, the Escorial in Spain, and the Palazza Ducale in Venicecould rival it for scale or magnificence.
(A) could rival it for - looks perfectly fine
(B) were the rivals for it in their - were denotes change of tense.
(C) were its rival as to - same as B
(D) could be its rivals in their - 'their' is ambiguous
(E) were rivaling its - same as B
Kudos
...
