The guiding principles of the tax plan released by the Treasury Depart

two main points of comparison are tested here.

first point.

if the main clause contain both subject and other element, such as object or complement (forget the grammatical terms. think that there is another term besides the subject), there is a chance of ambiguity. this means the comparison can be between two subject or two other elements

the principles have greater importance than the particulars of the plan

the main clause has 2 elements, subject "principles" and "importance". if the second part of comparison dose not contain do/did or helping verb, the ambiguity can happen. in above sentence , two meaning can be understood. principle have greater importance than the particulars do. or, principle have importance greater than the particulars.

if there is only subject. no ambiguity happens

I am taller than he.

this if fine, we dont need "he is" because there is no ambiguity.

second point

this is about do/did or helping verb.

if we see a verb phrase contain 2 part , "could/may/might + another form of verb", the second part comparison can have 2 meanings , meaning of could/may /might and meaning of the the other form of verb.

the principle could have greater significance than the particular of the plan could have
the priciple could have greater sigificance than the particulars of the plan do ̣̣(have)

we need could/may/might , helping verb or do/did, to make which meaning you want to show.

so, 2 elements in the main clause or 2 parts in the verb phrase can make 2 meanings. make meaning clear by adding do/did or helping verb could/may/might /have

going to answer choices.
choice c and d are wrong because "have" in the second part of comparison is helping verb. we need a form of "to have" , maybe in different tense, in the first clause to make "have" correct. for example

I had learned gmat like you have.

this is correct. "have" in the second part is good because we have "had" in the first part.

if the second part of comparison contain helping verb, the first part of comparison must contain the same helping verb, maybe in different tense. this is what Ron said

if we want "have" in choice c and e work as an action verb, we must replace "have" with "do/did". choice d do this job.

choice a and b are wrong because there are more than 1 elements in the first part and so, this make ambiguity. when we see many elements, find do/did or helping verb to make meaning clear. this rule makes us quickly find the errors