C :

#include<stdio.h>
#include<string.h>
int main()
{
    char a[25],b[25];
    int n,m;
    scanf("%d",&n);
    while(n--)
    {
        scanf("%d",&m);
        int max=0;
        while(m--)
        {
            scanf("%s",a);
            if(strlen(a)>max){ max=strlen(a);memset(b,'\0',sizeof b);strcpy(b,a);}
        }
        printf("%s\n",b);
    }
    return 0;
}

C++ :

#include<cstdio>
#include<iostream>
#include<cstring>
using namespace std;
int main()
{
	//freopen("test0.in","r",stdin);
	//freopen("test0.out","w",stdout);
	int n,m,max,i,len;
	char str[21],maxstr[21];
	cin>>n;
	while(n--)
	{
		cin>>m;
		max=0;
		for(i=0;i<m;i++)
		{
			cin>>str;
			len=strlen(str);
			if(len>max)
			{
				strcpy(maxstr,str);
				max=len;
			}
		}
		cout<<maxstr<<endl;
	}
	return 0;
}