Take this line from every branch and put it at the end of the loop:

printf("%c", password[i]);

Take this from every branch and put it at the start of the loop (and delete it from before the loop):

randomizer = rand() % 4;

This makes the loop look like:

for (i=0;i<n;i++)
{
    randomizer = rand() % 4;
    if(randomizer == 1)
    {
        password[i] = numbers[rand() % 10];
    }
    else if (randomizer == 2)
    {
        password[i] = symbols[rand() % 26];
    }
    else if (randomizer == 3)
    {
        password[i] = letterr[rand() % 26];
    }
    else
    {
        password[i] = letter[rand() % 21];
    }
    printf("%c", password[i]);
}

Take this line from every branch and put it at the end of the loop:

printf("%c", password[i]);

Take this from every branch and put it at the start of the loop (and delete it from before the loop):

randomizer = rand() % 4;

As Schwern observes, you can also turn the if chain into a switch/case, which is a bit faster and easier to read.

This makes the loop look like:

for (i=0;i<n;i++)
{
    randomizer = rand() % 4;
    switch (randomizer) {
    case 1:
        password[i] = numbers[rand() % 10];
        break;
    case 2:
        password[i] = symbols[rand() % 26];
        break;
    case 3:
        password[i] = letterr[rand() % 26];
        break;
    default:
        password[i] = letter[rand() % 21];
        break;
    }
    printf("%c", password[i]);
}

Take it from everywhere to the end of the cycle:

 printf("%c", password[i]);

Take it from everywhere to the begin of the cycle(and delete it from begin of the program):

randomizer = rand() % 4;

Take this line from every branch and put it at the end of the loop:

printf("%c", password[i]);

Take this from every branch and put it at the start of the loop (and delete it from before the loop):

randomizer = rand() % 4;

This makes the loop look like:

for (i=0;i<n;i++)
{
    randomizer = rand() % 4;
    if(randomizer == 1)
    {
        password[i] = numbers[rand() % 10];
    }
    else if (randomizer == 2)
    {
        password[i] = symbols[rand() % 26];
    }
    else if (randomizer == 3)
    {
        password[i] = letterr[rand() % 26];
    }
    else
    {
        password[i] = letter[rand() % 21];
    }
    printf("%c", password[i]);
}