Identify the element with the least atomic radius among $\text{Li, Na, Be, Mg, B, Al}$.

Atomic radius generally:

• decreases across a period from left to right,
• increases down a group.

Now compare the given elements:

• Period 2: $\text{Li, Be, B}$
• Period 3: $\text{Na, Mg, Al}$

Since atomic radius decreases across a period, in period 2: $\text{Li} > \text{Be} > \text{B}$ and in period 3: $\text{Na} > \text{Mg} > \text{Al}$

Also, period 2 elements are generally smaller than corresponding period 3 elements.

Therefore, the smallest atomic radius among all given elements is for: $\boxed{\text{B}}$

Find the oxide formed by boron.

Boron commonly forms the oxide: $\text{B}_2\text{O}_3$

This matches the general formula: $\boxed{A_2O_3}$

Match with the options:

• A: $AO$
• B: $AO_2$
• C: $A_2O$
• D: $A_2O_3$

Hence the correct option is: $\boxed{\text{D}}$