The electrode potential of $M^{2+} / M$ for $3d$-series elements shows a positive value for which of the following?

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:

Which of the following statements is not correct? $(Ve = 58, \, Lu = 71, \, La = 57, \, Yb = 70)$

$A$ transition metal $X$ has a configuration $[Ar]3d^4$ in its $+3$ oxidation state. Its atomic number is

Refractory metals are used in the construction of furnaces because...

Many transition metals form interstitial compounds. The characteristics of these interstitial compounds are:
$I$. They have high melting points,higher than those of pure metals.
$II$. They are very hard.
$III$. They retain metallic conductivity.
$IV$. They are chemically more reactive than the pure metals.