The GMAT tends to write sentences in which "which"stands for the ELIGIBLE noun that's closest to the comma. By "eligible", I mean that the noun has to agree in terms of singular/plural with the following verb.
Here's an example:
The box of nails, which is on the counter, is to be used on this project.
In this case, "which" CANNOT refer to "nails". So, "which" unambigously stands for that.
In my observation, the GMAT has been VERY good about this.
Whenever I've seen a "which" that refers to "X + preposition+Y" rather than just Y, it has always been the case that X was singular and Y was plural or vice versa, and the verb had a form that matched X and didn't match Y.
So be cautious about this fact and rest assured that it will pay off very soon. :D
Good observation. I would like to mention the grammatical explanation to above.
Subjects do not appear in "Prepositional Phrase" eg. of nails
. They are called the "middleman" to hide the real subject.
Exception: In case of Some Any None All More/Most " SANAM" Pronouns and quantity modifiers like majority, half etc the noun in the prepositional phrase is considered as the subject and decides the number of the verb.