The answer is $\boxed{1,6}$.
The missing number is the product of the first two numbers and the sum of the last two numbers. In other words, the missing number is equal to $a_1a_2 + a_3a_4$, where $a_1$ and $a_2$ are the first two numbers and $a_3$ and $a_4$ are the last two numbers.
In this case, the first two numbers are $1$ and $6$, and the last two numbers are $8$ and $1$. Therefore, the missing number is equal to $1 \times 6 + 8 \times 1 = 16$.
Option A is incorrect because $6 \times 1 + 8 \times 1 = 15$.
Option B is incorrect because $8 \times 1 + 1 \times 1 = 9$.
Option C is incorrect because $1 \times 6 + 1 \times 8 = 15$.
Option D is incorrect because $1 \times 8 + 1 \times 1 = 9$.