Rearrange the parts of the sentence in correct order.
The fundamental cause of
P-modern world the stupid are cocksure
Q-the trouble is that in the
R-while the intelligent are full of doubt

The sentence segment starts with "The fundamental cause of...".
It needs a noun phrase to follow "of", which is provided by Q ("the trouble is that in the").

"in the" in segment Q must be followed by a noun phrase specifying a place or era, which is found in P ("modern world...").

Segment P finishes with a statement about "the stupid", and segment R provides the contrasting clause ("while the intelligent are...").
Thus, the correct order is QPR, making the complete quote by Bertrand Russell: "The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt."