Select the related word/letters/number from the given alternatives.17 : 493 :: ? : 551

Let's look at the relationship between the first pair:
$$\frac{493}{17}=29$$
So, $17\times 29=493$.Using the same logic for the second pair, let the missing number be $x$:
$$x\times 29=551$$
$$x=\frac{551}{29}=19$$
Therefore, the missing number is 19.