I was solving a couple of questions, and this one really intrigued me:
The police resorted to lathi charge ____ extreme provocation.
("lathi charge" is a form of police control employed in India to stop a mob from going rampant)
What preposition should fit into the blank?
More specifically, should it be 'upon', or should it be 'under'?
You want to use "under" in this case. The following definition of under is found at The MacMillan Dictionary, definition 3...
in the process of being affected or controlled by a particular action, situation, or state
So in the example, police resorted to lathi charge because they were "being affected or controlled by" the extreme provocation of the crowd. They were under extreme provocation.
As a native speaker, it's hard to explain exactly why "upon" is not correct. I can only offer intuitive reasoning. "Upon" has the same meaning as "on," so in this situation it would imply that the deciding factor in implementing lathi charge was controlled by the police. It wasn't. The actions of the mob, in a sense controlled the decision of the police. Without really changing the meaning, the idea could be reworded to use upon by changing the subject from the police to the crowd, like so...
The effect of the crowd's extreme provocation upon the police was the implementation of lathi charge. This is not nearly as concise or clear, but it illustrates that the crowd has the upper hand.
I agree with Stephen C's analysis above, especially the suggestion to reword the sentence. Here is an option that's very explicit about who was provoked:
The police, being [feeling?] subject to extreme provocation, resorted to [a] lathi charge.
Or, more casually:
The police were [felt?] subject to extreme provocation, and resorted to [a] lathi charge.
