@ And I forgot to tell you that the background (at the bottom) was deleted here.

(And I mixed up the origins of P40 and P41 in the notes before. Fixed.)

@

A bucket sort is enough.

#include <bits/stdc++.h>
using namespace std;
int a[4512], b[8912];
int main()
{
    int n, m, k;
    cin >> n >> m >> k;
    for (int i = 1; i <= n; i++)
    {
        int x;
        cin >> x;
        a[x]++;
    }
    for (int i = 1; i <= m; i++)
    {
        int s;
        cin >> s;
        while (s--)
        {
            int x;
            cin >> x;
            b[x] += a[i];
        }
    }
    int mp = 0, mi = 0;
    for (int i = 1; i <= k; i++)
        if (b[i] > mi) mi = b[i], mp = i;
    cout << mp << endl;
    return 0;
}