Your bibliography need not cite any source for the sample test you adapted—not “adopted”—from the Forum website. You can be accused of plagiarizing a certain test only when you copy it word for word without getting permission from the official test maker. You see, good sample tests just faithfully follow or emulate the pattern of known official sample tests of TOEFL, TOIEC, or IELTS. They just change the names and particulars used in those tests, staying as close as possible to the patterns and difficulty levels set for each test item. In fact, test developers or writers that attempt to create totally new test patterns, or those that depart significantly from official test samples just for novelty or originality or creativity’s sake, are actually doing it all wrong. This is because every test passage and test question in official tests follow very precise language levels and exacting testing criteria for such aspects as grammar, sentence construction, syntax, and idiom for the different reading and listening comprehension levels or writing skill levels. Indeed, the more faithful or closer a sample test is to these criteria, the more useful it will be to those who use or take it for exercise or familiarization purposes.
You said that you were advised by some panelists evaluating your dissertation that you need to rephrase the question in this particular test item:
One magazine has declared Dan Brown as what?
a. among the 100 wealthiest people.
b. among the 100 most influential people.
c. more popular than J. K. Rowling.
d. more prolific than J. K. Rowling
I think they did so because in actual official tests, it’s extremely rare for questions to be phrased in “hanging form” like the one you presented above, with the question ending with the interrogative “what?” and the answer choices presented in serial, enumerative fashion after that. On extremely rare occasions, the official test makers may do it just to create “distractors” or “distracters,” or perhaps simply to “mark off” or identify a particular test in a series they have produced, but that’s an idiosyncratic liberty only they as the official test producers can do. In any case, a simple structural fix for that question is to just drop “what?” so that it becomes a simple declarative statement followed by the answer choices, as follows:
One magazine has declared Dan Brown as
a. among the 100 wealthiest people.
b. among the 100 most influential people.
c. more popular than J. K. Rowling.
d. more prolific than J. K. Rowling.
Note that in this latter test construction, each answer choice ends with a period or full stop--clearly indicating from a sentence structure standpoint that the phrase or clause is simply completing the opening statement of the test.
I hope these observations of mine will be of help to you.