PTA.S1.Q3 - In a democratic society, when a political interest group...

I chose B with certainty in my blind review, but found out that D was the correct answer.

Is the answer choice B not the correct answer because it does not mention the "certain size" of political interest group?

Also, if D is the correct answer, does it mean that "as wide a membership as possible" is supported by what the passage says "exceeds a certain size"?

Thank you in advance!