Identify the trend involved

Group 13 elements here are $\mathrm{Ga}$, $\mathrm{In}$ and $\mathrm{Tl}$.

They commonly show oxidation states $+3$ and $+1$.

Understand the inert pair effect

Down the group, the outer $ns^2$ electrons become increasingly reluctant to participate in bonding. This is called the inert pair effect.

Hence, the lower oxidation state becomes more stable down the group.

So for Group 13: $+1\text{ state stability increases down the group}$ Therefore, $\mathrm{Ga}(+1) < \mathrm{In}(+1) < \mathrm{Tl}(+1)$

Check Assertion A

Assertion A states that the stability order of $+1$ oxidation state of $\mathrm{Ga}, \mathrm{In}$ and $\mathrm{Tl}$ is increasing down the group, i.e. $\mathrm{Ga} < \mathrm{In} < \mathrm{Tl}$ This is true.

Check Reason R

Reason R says: The inert pair effect stabilizes the lower oxidation state down the group.

This is exactly the known reason for the increasing stability of the $+1$ state in Group 13.

So R is true.

Check whether R explains A

Since the inert pair effect is the reason why $+1$ oxidation state becomes more stable from $\mathrm{Ga}$ to $\mathrm{Tl}$, R is the correct explanation of A.

Choose the correct option

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

Compare with stored answer

Stored correct answer = $\text{D}$.

My derived answer also = $\text{D}$, so they agree.