Since Senator Johnston revised the estate tax code

A) Since Senator Johnston revised the estate tax code in a way that allows grandchildren living outside the United States to claim special exemptions, many of them moving abroad with the expectation of
many of them moving abroad is missing a main verb therefore out

B) Because Senator Johnston's revisions to the estate tax code allow grandchildren living outside the United States to claim special exemptions, many of these descendants have moved abroad, expecting
THe verb tense is perfect with the timeline , something that has happened in the past is continuing to the present right use of 'have' , the meaning is perfect therefore let us hang on to it

C) Grandchildren living outside the United States can claim special exemptions, according to Senator Johnston's revisions to the estate tax code, and so many have moved abroad and these descendants are expecting
'these descendants ' is unnecessary usage since the refernce is perfectly portrayed in the sentence above along with that there is no real need to change the tense of the sentence from persent perfect to present therefore out

D) The revisions to the estate tax code, written by Senator Johnston, allow grandchildren living outside the United States to claim special exemptions, so many of them had moved abroad, anticipating
This gives us a meaning the estate tax code was not amended by Senator Jhonson ,rather just written clearly not the intended meaning therefore out

E) Because Senator Johnston had revised the estate tax code, which allows grandchildren living outside the United States for claiming special exemptions, which causes them to move abroad, in anticipation of
The verb tense issue here is a problem since 'had' is used without any simple past events therefore out

THerefore IMO B

The long underline signifies that there is a structural problem with the sentence and thats what we should focus on.
A) Since Senator Johnston revised the estate tax code in a way that allows grandchildren living outside the United States to claim special exemptions, many of them moving abroad with the expectation of a large inheritance. --> This does not have a complete core sentence. Here we have two fragments. First is "Since Senator Johnston revised the estate tax code in a way that allows grandchildren living outside the United States to claim special exemptions" and second is "many of them moving abroad with the expectation of a large inheritance."

B) Because Senator Johnston's revisions to the estate tax code allow grandchildren living outside the United States to claim special exemptions, many of these descendants have moved abroad, expecting a large inheritance. --> there is no error --> CORRECT

C) Grandchildren living outside the United States can claim special exemptions, according to Senator Johnston's revisions to the estate tax code, and so many have moved abroad and these descendants are expecting a large inheritance. --> "according to Senator Johnston's revisions to the estate tax code" should come before "Grandchildren living outside the United States can claim special exemptions" --> Incorrect

D) The revisions to the estate tax code, written by Senator Johnston, allow grandchildren living outside the United States to claim special exemptions, so many of them had moved abroad, anticipating a large inheritance. --> "written by Senator Johnston" --> Did he write the code himself --> Changes the meaning of the sentece --> Incorrect

E) Because Senator Johnston had revised the estate tax code, which allows grandchildren living outside the United States for claiming special exemptions, which causes them to move abroad, in anticipation of a large inheritance. --> Grandchildren are moving not because Senator Johnson has revised the code but because tax code allows grandchildren exemption. --> Changes meaning --> Incorrect