Complete the pattern

$1 \times 1=1$

$11 \times 11 = 121$

$111 \times 111 = 12321$

$1111 \times 1111 = \_\_\_\_\_\_\_\_\_$

Following the given pattern, we can see that,

$1 \times 1=1$

$11 \times 11 = 121$

$111 \times 111 = 12321$

Then for $1111 \times 1111$, as per the same trend, the answer is $1234321$. Also, if we multiply $1111 \times 1111$, we will get $1234321$.

Hence, the correct answer is option D.