Read the sentence to find out if there is any grammatical error or idiomatic error in it. The error, if any, will be in one part of the sentence. Mark that part as the answer. (Ignore errors of punctuation, if any.)
The devastating experiences /of many wars taught some countries /the necessary of pursuing peace /at the expense of nationalist egos.
the necessary of pursuing peace
Here, the sentence is erroneous because of wrong usage of adjective, ‘necessary’, which is an adjective meaning ‘pursuing peace’ should be replaced with ‘necessity’ (noun).