Select the related number from the given alternatives.
$$2.3:8::3.4:?$$

Let us analyze the relationship in the first pair $2.3:8$.
The decimal notation represents the mathematical power rule: $a.b⟹a^b$.

Applying this to the first term:

$$2^3=8$$

Following the same pattern for the second pair $3.4:?$:

$$3^4=3\times 3\times 3\times 3=81$$