In which of the following is the metallic bond strongest?

Which of the following oxides is white but becomes yellow on heating?

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:

German silver is an alloy of $Cu$ with

Which pair of metal ions from the following has the same number of unpaired electrons?

The atomic radii of the elements are almost the same for which series?