Find out the alternative which will replace the question mark (?):\n
$$138:18^3::723:?$$

Let's analyze the digital pattern of the first pair $138:18^3$:
The first digit ($1$) and the third digit ($8$) combine to form the base number $18$. The middle second digit ($3$) forms the exponent power.

$$\text{Pattern: }\overline{xyz}⟹(xz{)}^y$$

Applying this identical pattern to $723$:

* First digit $x=7$
* Third digit $z=3$
* Second digit $y=2$

Combining them gives the base $73$ raised to the power of $2$, which is $73^2$.