2 things.

Consider X Y is the correct idiom. You can this eliminate B,C and E which used considered as.

THis is a case of superficial parallelism, setting free _ _ _ _ occurred when he filed a deed of emancipation.

comma+ ing ... this works as a modifier

Read it like this..

"setting free the 500 slaves, Robert stunned his friends"

Also remember to cut the modifier/definition crap in between comma's.
Please do not read " one of the rich plantation owner in virgina".. waste of words..

Can some please tell the answer with proper reasoning??

The official ans given in OG 10 is as under:-

This sentence requires that the participial phrase setting free... connect to the gerund construction by filing a
deed...; it was the filing of a deed that made possible the setting free .... Choices A and B establish this
connection, but only A, the best choice, completes the participial phrase appropriately. In choices B and D the
misconstructed phrases set[ting] free more than the 500 slaves ... mistakenly suggest that Carter set free
slaves that were not his own. Choices C and D distort meaning by paralleling stunned and set free, as though
these were two separate and independent actions. E begins a second independent clause, which--though
grammatically acceptable--again distorts the meaning. In choices B, C, and E, considered as is unidiomatic.

filing and setting are parallel words therefore left with A and B..........

considered as is wrong in B therefore left with option A

Choice E uses and (a coordinating conjunction). Almost always, and connects two unrelated facts (two Independent clauses). For example:

My name is James and my girlfriend's name is Megan.

My name and my girlfriend's name are two unrelated facts. "and" is a perfect usage of joining such facts. In light of this, in the sentence under consideration, E seems to suggest the following two "unrelated" facts:

1. Robert Carter stunned his family, friends, and neighbors by filing a deed of emancipation
2. Robert Carter set free the more than 500 slaves who were legally considered his property

But then, logically, the above two are not unrelated events; the deed of emancipation resulted in 500 slaves being set free. This relationship does not come out when "and" is used (as in case of E).

While this specific sentence can most easily be answered if one were to know that considered as/considered to be etc. are all not valid idioms, going forward we are unlikely to find questions on GMAT that can be solved on the basis of Idioms alone.