Going with B.

Agree. With this reasoning I also opted for B. But I was surprised to see that OA is not B.

Two things basically - If the question is just about without any permission and without permission, any just acts as an intensifier of sorts - makes it more stark.

However if you were confused between B and C, then C simply changes the meaning.

which no commoner or foreigner could enter without permission, on pain of death.

on pain of death as stated above does NOT convey the meaning that the foreigners or commoners COULD ENTER WITHOUT PERMISSION ONLY ON PAIN OF DEATH.

Let's assume C is the "correct" answer. Now, let's examine the sentence after grafting the clause presented in C:

The Forbidden City in Beijing, from which the emperors ruled by heavenly mandate, was a site which no commoner or foreigner could enter without permission, on pain of death.

Now if we remove the non-restrictive clauses connected by which and let's see what we get :

The Forbidden City in Beijing was a site on pain of death. - Does this make any sense ?
_________________

"Education is what remains when one has forgotten everything he learned in school."

Wouldn't the sequence of steps applied above for B lead to the same absurdity? How is B different from C in this case?

I think C is actually correct after all.

On pain of death is a prepositional phrase which acts as an adverb - also, it acts as a DISJUNCTIVE ADVERB.

You call can read about disjunctive adverbs at the following link.

http://grammar.ccc.commnet.edu/grammar/adverbs.htm

The essential issue between B and C is recognizing that "on pain of death" is a disjunctive adverb modifying "enter" - In B, "only" acts as an adverb modifying another adverb on "pain of death"; "only" is NOT entirely indispensable and dropping it from the sentence wouldn't create a wrong sentence (as happens in C).

(B) which a commoner or foreigner could enter without any permission only

(C) which no commoner or foreigner could enter without permission,

C has the advantage that it conveys the information in an active sense - No one could enter without permission versus One could enter without permission

So C is active and concise and hence I think should fine.

By the way - I have done quite a few questions from 1000 SC and I think most of the answers are correct. At least I have not encountered any wrong answers so far. But then again I have not completed all the questions and I may not have understood a few answers that were actually wrong that I thought were correct.

First of all let's evaluate the role of ONLY in the following sentence - I hope we all agree that ONLY acts as an adverb and as such it cannot be critically essential to the grammatical correctness of any sentence. Adverb can only qualify a verb. So dropping it would not create a grammatical flaw.

One could enter without (any) permission only on pain of death

dropping only from the sentence we get

One could enter without (any) permission on pain of death.

Now what is the difference between the above statement and the one in C?

No one could enter without permission, on pain of death

Only the word order - No one could enter and One could enter are changed. Both convey the same meaning.

In other words C expresses the idea more compactly.


Also -

No one could enter without permission, on pain of death

Does this mean you need permission to enter on pain of death?

Or that if you did enter without permission it would be on pain of death?

For it to convey what you are saying - it would need to be reworded slightly

No one could, without permission, enter on pain of death

Then it would mean you need permission to enter on pain of death.

OK..before somebody else points out - I will admit that besides acting as an adverb ONLY can also act as an Adjective (as in the ONLY Child) and a Conjunction (You can certainly go, only be careful)