Find out the alternative which will replace the question mark (?).
$$243:234::621:?$$

Let's analyze the relationship in the first pair $243:234$:
The digits of the number $243$ are rearranged. The first digit remains in place, while the second and third digits exchange positions:

$$2\to 2$$

$$4\to \text{moved to end}$$

$$3\to \text{moved to middle}$$

So, $243\to 234$.

Applying the same digit rearrangement pattern to $621$:
The first digit ($6$) stays in place, while the second digit ($2$) and third digit ($1$) swap places.

$$6\to 6$$

$$2\to \text{moved to end}$$

$$1\to \text{moved to middle}$$

Thus, $621\to 612$.