agni_mba

The rule to remember is that a collective noun is always singular. Congress is a collective noun and hence using 'was' is correct. I think the rule applies to 'jury' too

Prasanna

Thanks Prasanna for reverting back.

Quoting Wren and Martin "if the collective noun conveys the idea of separate individuals comprising the whole, the pronoun/verb standing for it must be of the Plural Number: as, the jury were divided in their opinion".

Is it because of the minor differences between GMAT English and "High-school English?

The usage of was/ were in a sentence especially when they are subjected to a group of people.
was is the past tense of is and a singular verb.
were is the past tense of isand a plural verb.
let's see how this expression of was and were apply to this option.

option A does not give the require subject-verb agreement

option B is incorrect. "members was" cannot go together. This is because they are wrong combination of words, members is in plural form and it must attract a plural verb were and not was

for option C, "were unanimously approving a law aimed at reducing the flow of junk e-mail, there were members". the usage of there makes the option incorrect because it does not conform with the question statement.

in option E, the usage of was makes the option incorrect because the plural form of members should be used with a plural verb were. Also the use of there do not comply with the statement in the question

in option D, this is the correct option because it contains a plural verb were which fit in with members. 'unanimously' makes the use of were comes into play because it suggest a congress of more than one person.
The members were from the question is the right noun-verb concord.