Which among the following elements has the lowest density and is the lightest?

The correct set of ions (aqueous solution) with the same colour from the following is:

The number of $4f$ electrons in the ground state electronic configuration of $Gd^{2+}$ is ........ .
[Atomic number of $Gd=64$]

Identify the oxidation states of titanium $(Z=22)$ and copper $(Z=29)$ in their colourless compounds.

Match the following:

List-$I$ (Element)	List-$II$ (Block)
$A$. $Cd$	$I$. $f$-block
$B$. $Eu$	$II$. $s$-block
$C$. $Se$	$III$. $d$-block
$D$. $Ba$	$IV$. $p$-block

The correct answer is:

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$. Assertion $(A)$: In aqueous solutions $Cr^{2+}$ is reducing while $Mn^{3+}$ is oxidising in nature.
Reason $(R)$: Extra stability to half-filled electronic configuration is observed than incompletely filled electronic configuration.
In the light of the above statement,choose the most appropriate answer from the options given below: