Why is the correct option D and not option B?

#1

I am not able to understand how can we represent the undirected graph having n nodes using a vector of type int of size n+1

#2

ans is D because all options are correct , let me explain how,

  1. vector < int > graph [ N + 1 ]
    Here what is happening is we are declaring an array of vector and the array size is N+1 .
    how?? , when we do int arr[N+1] it means an array of type int with size N+1 similary if you replace int with vector , you will get vector arr[N+1] , thus u will get an array of type vector with size N+1. so you got a 2D data structure which can be used to implement adjacency list

  2. since you think b is the answer then i am assuming you know why

  3. vector < vector < int > > graph(N+1) here we are declaring a vector of type vector of size N+1 , which is again a 2D structure and can be used to represent adjacency list

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#3

Thanks for the excellent clarification. Cheers!