This is the meaning issue. Gregory the Great is the name of a king.

b) who was the last one of the Pictish kings => This choice makes a sense that Gregory the Great is the last one of Pictish kings. Gregory the Great sounds like thing, not human being. If translate in the human sense, Gregory the Great was the last king of the Pictish kings, still sound like things or be redundant. In this case, the plural "kings" must be removed.
c) who was one of the last Pictish kings => sound more correct.
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I too rejected C on the same grounds.

Can you please share the source of the Question. On one hand, you write That the question is from GMATCLUB tests and on the other hand this question is tagged under the source name GMATPREP test. Please confirm.

Also this Question has never been discussed anytime before this.

Totally agree with you guys. I chose B for the same reasons.

Ok, thanks for the answer. Great explanation concerning the "one" that should not be employed to say that Gregory was the last and not one of the last.

But do you think the GMAT could give a question like that on the exam? Wouldn't it be too confusing for exam takers?

Not yet convinced with (C). I go with(B).

"the last one of the Pictish kings" - there may be better way of telling this, but I don't believe option is grammatically incorrect.

I believe when you fall in the dilemma to choose an option between :

1) Wordy but grammatically correct, but keeps the meaning of the original sentence intact
2) Grammatically correct, not wordy , but changes the meaning


Then I choose (1). Because to me ,retaining the meaning of the sentence is more important than a wordy sentence ( provided that the wordy sentence is grammatically correct)

Please post our suggestions on my explanation
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Thank you, jumsumtak, for the response. I apologize as I am replying after a few months of dormancy.
As I see, your second example makes sense if Y is plural.

Here, Pictish Kings means one particular dynasty of rulers. There were many successive rulers who fall under the category "Pictish Kings". It is ok to say "Gregory was last one of the Pictish Kings" (which is intended to convey that "Gregory was the last Pictish King").

There is little difference between meaning of option B and meaning of option C. Option B means Gregory was the last Pictish King and option C means Gregory was one of the last Pictish kings. Following 2 scenarios will clarify the concept.

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Scenario 1: Assuming chronological order of the Pictish Kings was -
Bridei, Ciniod, Alpín, Talorgan, Dubhtolargg, Drest, Conall, Caustantín, Óengus, Drest, Gregory, Eógan

Here Gregory was not the last Pictish King, but he was among the last two Pictish Kings. In this case, it would be wrong to say "Gregory was the last one of the Pictish kings" (option B), but it would be right to say "Gregory was one of the last Pictish kings" (option C).

Scenario 2: Assuming chronological order of the Pictish Kings was -
Bridei, Ciniod, Alpín, Talorgan, Dubhtolargg, Drest, Conall, Caustantín, Óengus, Drest, Eógan, Gregory

Here Gregory was the last Pictish King and thus he was among the last two Pictish Kings. In this case, it would be right to say "Gregory was the last one of the Pictish kings" as well "Gregory was one of the last Pictish kings".
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Original sentence meant Gregory was the last Pictish King and same thing is conveyed in option B. Option C has a little change of meaning as we cannot get any information on whether Gregory was the last Pictish King or he was last but one Pictish King.

Problem with option B is that the construction "Gregory was the last one of the Pictish kings" is wordy. Economic expression would have been "Gregory was the last Pictish king".

To compare:
Option B: retains meaning (+ point) but wordy (- point)
Option C: changes meaning (- point) though concise (+ point)

IMHO, B wins over C as B retains the meaning and C twists the meaning subtly.
However, considering the OA, it seems style is more important than meaning.
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