Which one of the following belongs to the $3d-$ transition series?

Stainless steel does not rust because

The electronic configuration of gadolinium (atomic no. $64$) is

Assertion : $Cr(VI)$ in the form of dichromate in acidic medium is a strong oxidising agent whereas $MoO_3$ and $WO_3$ are not.
Reason : $Mo(VI)$ and $W(VI)$ are found to be more stable than $Cr(VI)$.

Which of the following pairs has approximately the same ionic size?

Which of the following statements is not true about interstitial compounds?