Use the inert pair effect in group 13

Down group 13, the stability of the lower oxidation state $+1$ increases due to the inert pair effect.

• For aluminium, the most stable oxidation state is $+3$.
• For thallium, the oxidation state $+1$ is more stable than $+3$.

Check statement (A): $Tl^{3+}$ is a powerful oxidising agent

Since $Tl^{3+}$ tends to get reduced to the more stable $Tl^+$ state, it readily accepts electrons.

Therefore, $Tl^{3+}$ acts as a strong oxidising agent.

So, (A) is correct.

Check statement (B): $Al^{3+}$ does not get reduced easily

Aluminium strongly prefers the $+3$ oxidation state. Reducing $Al^{3+}$ to a lower oxidation state is difficult.

Hence, $Al^{3+}$ does not get reduced easily.

So, (B) is correct.

Check statement (C): Both $Al^{3+}$ and $Tl^{3+}$ are very stable in solution

• $Al^{3+}$ is stable.
• But $Tl^{3+}$ is not very stable in solution because it tends to reduce to $Tl^+$.

Therefore, (C) is false.

Check statement (D): $Tl^{1+}$ is more stable than $Tl^{3+}$

This is a direct consequence of the inert pair effect in thallium.

So, (D) is correct.

Check statement (E): $Al^{3+}$ and $Tl^{1+}$ are highly stable

• $Al^{3+}$ is the stable oxidation state of aluminium.
• $Tl^+$ is the stable oxidation state of thallium.

Therefore, (E) is correct.

Collect correct statements

Correct statements are: $A, B, D, E$

Match with options

Option A says: $(A), (B), (D)$ and $(E)$ only.

Hence, Option A is correct.