Sequencing

The pattern is: $n_1 + n_2 = n_3$ and $n_1 \times n_2 = n_4$ $~ ~ ~ ~ ~ ~ ~ ~$ $;[n_1 = \text{first number}, n_2 = \text{second number} \ldots.]$

A: $5+7 = 12 \implies 5 \times 7 = 35$

B: $8+9 = 17 \implies 8 \times 9 = 72$

C: $12 + 7 = 19 \implies 12 \times 7 = 84$

D: $3 + 13 = 16 \implies 3 \times 13 = \boxed{39}$

Let the first two terms of each sequence be $a$ and $b$

Then the third term is $a + b$ and the fourth term is $a \times b$

$3 \times 13 = 39$