- AChromium
- ✓Scandium
- CNickel
- DCopper
Correct option is B. scandium
The $d-b l o c k$ elements that are the elements from a group $3-12$ are called transition elements. This is because their physical and chemical properties are intermediate between $s-b l o c k$ and $p-$ block properties. Hence these elements act as a bridge from $s-b l o c k$ to $p-b l o c k$ and hence they are known as transition. They have several oxidation states. They usually form colored compounds. An atomic number of these metals is from $21-30,39-48,57,72-80,89$, and $104-112$.
$\because$ The First main series of Transition elements starts with Scandium(Atomic no $=21)$ end with $\operatorname{Zinc}($ Atomic no $=30)$
$\therefore$ The first element of transition element is Scandium
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$(A)$ The total number of stereoisomers possible for ${X}$ is $6$
$(B)$ The total number of diastereomers possible for ${X}$ is $3$
$(C)$ If the stereochemistry about the double bond in ${X}$ is trans, the number of enantiomers possible for ${X}$ is $4$
$(D)$ If the stereochemistry about the double bond in ${X}$ is cis, the number of enantiomers possible for ${X}$ is $2$