Metal-metal bonding is more frequent in $4d$ or $5d$ series than in $3d$ series due to

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: In $T \ell I_{3}$,isomorphous to $CsI_{3}$,the metal is present in $+1$ oxidation state.
Reason $R$: $T \ell$ metal has fourteen $f$ electrons in the electronic configuration.
In the light of the above statements,choose the most appropriate answer from the options given below:

Which of the following is not correct about transition metals?

Identify the ferromagnetic element from the following.

The electronic configuration of $Pt$ (atomic number $78$) is $...$

Effective magnetic moment of $Sc^{3+}$ ion is