How can I pass values from different multiple arrays to a new single array?

My code:

void Block::word()
{
    std::string worda[] = {"cat","owl","cow"};
    std::string wordb[] = {"apple","power","block"};
    std::string wordc[] = {"account","extreme","kingdom"};
    std::string wordd[] = {"adjective","undefined","pineapple"};

    srand(time(0));
    fnwords[0] = worda[rand() % 3 + 1];
    fnwords[1] = wordb[rand() % 3 + 1];
    fnwords[2] = wordc[rand() % 3 + 1];
    fnwords[3] = wordd[rand() % 3 + 1];

    for (int d=0; d<4; d++){
          std::cout << fnwords[d] << std::endl;
      }
}

int main()
{
    Block obj;
    obj.word();
    return 0;
}

Here the first index of array fnwords must contain a random word from array worda, second index from wordb, and so on.

But it gives error sometimes that program.exe has stopped working.

And it initializes array like this:

-cow
-cat
-kingdom
-undefined