Analyze Assertion A

The compound $TlI_3$ is said to be isomorphous with $CsI_3$.

• In compounds like $CsI_3$, the formulation is better written as: $Cs^+[I_3]^-$
• Here, the anion is the triiodide ion, $I_3^-$.

So, similarly, $TlI_3$ being isomorphous with $CsI_3$ means it is actually: $Tl^+[I_3]^-$

Therefore, thallium is present in the +1 oxidation state, not +3.

Hence, Assertion A is correct.

Analyze Reason R

Electronic configuration of thallium ($Z=81$) is: $[Xe]4f^{14}5d^{10}6s^26p^1$

So thallium indeed has fourteen $f$ electrons.

Hence, Reason R is also correct.

Check whether R explains A

The real reason for the stability of $Tl^+$ over $Tl^{3+}$ is the inert pair effect.

• In heavier p-block elements like Tl, the $6s^2$ electrons are reluctant to participate in bonding.
• This happens due to poor shielding by inner $d$ and $f$ electrons, which increases effective nuclear attraction on the $6s$ electrons.

Although the presence of $4f^{14}$ electrons is true and contributes indirectly through poor shielding, the statement "Tl metal has fourteen $f$ electrons" alone is not the proper explanation of why in $TlI_3$ thallium is effectively in the +1 state.

Thus, both A and R are correct, but R is not the correct explanation of A.

Correct option

$\boxed{C}$