@ soumita, This question exemplifies the importance of using logic of the sentence to understand the parallel structure. I believe you are facing problems in this question since you are focusing only on the grammar part - i.e. in this sentence you made "resulting" parallel to "causing" since both are verb-ing forms. But as you know this is not correct. So lets look at this sentence in detail.

The human resource department's decision to expand health care benefits to domestic partners of its employees will either be approved by the vice president,which results in an end to the boycott, and cause another protest.

A)which results in an end to the boycott, and cause
B)resulting in an end to the boycott, or causing
C)with the result of an end to the boycott, or causing
D)resulting in an end to the boycott, or cause
E)with a resulting end to the boycott, and cause

Understand Original Sentence:
The human resource department's decision to expand health care benefits to domestic partners of its employees will either be approved by the vice president,which results in an end to the boycott, and cause another protest.

This sentence talks about the two things that may happen after the HR decides to offer health care benefits to domestic partners as well.
1: The decision will be approved by the VP - this will end the boycott.
2: The decision will not be approved by the VP - this will cause another protest.

Determine the Errors in the Original Sentence
The human resource department's decision to expand health care benefits to domestic partners of its employees will either be approved by the vice president,which results in an end to the boycott, and cause another protest.

1: Which results...is a noun modifier and it incorrectly modifies VP. It should be a verb-ing modifier that shows the results of the preceding clause or verb.
2: "either" should be used with "or". Here it is used with "and".
Per the meaning of the sentence, parallel elements are as follows:
either be approved by the VP
or cause another protest

Process of Elimination

A)which results in an end to the boycott, and cause - Wrong as discussed
B)resulting in an end to the boycott, or causing - Wrong

• Modifier error corrected
  either..or error corrected
  Illogical list.

C)with the result of an end to the boycott, or causing - Wrong

• awkward construction - result of an end...
  Illogical list

D)resulting in an end to the boycott, or cause - Correct
E)with a resulting end to the boycott, and cause- Wrong

• resulting end is incorrect
  either..or error

Takeaway:
Make sure you understand the meaning of the sentence well enough and then determine the logically parallel entities. Once you determine that, you should concentrate on making the entities parallel.

Hope this helps.

Regards,

Payal

I am glad that this helped.

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Thanks,

Payal