Answer: Option A. -> Co(Z=27)       
:
A
Generally, standard reduction potential value increases from left to right in the period of d-block elements but there are some exceptions. Mn${}^{3+}$ and Co${}^{3+}$ behave as strongest oxidizing agent in aqueous solution.$E^{0}C{o}^{3+}/C{o}^{2+}$ = + 1.97V is highest suggesting thereby that Co${}^{3+}$ is most stable in aqueous solution. Thus, Co (Z=27) has the highest $E^{0}C{o}^{3+}/C{o}^{2+}$

Answer: Option A. -> Sc+3
:
A
$S{c}^{+3}⟶1s^{2}2s^{2}2p^{6}3s^{2}3p^6$
Due to presence of paired electron it will be colourless.

Answer: Option D. -> (n−1)d5 ns2
:
D
$(n-1)d^{5}n{s}^2$ can achieve the maximum oxidation state of +7.

Answer: Option C. -> Fe+3
:
C
$\begin{array}{ccccc}S.No.& Outerconfigu-ration& No.ofunpaired{e}^{–}& Colourofion& Magneticmoment\\ V^{+3}& 3d^2& 2& Green& 2.76\\ M{n}^{+3}& 3d^4& 4& Violet& 1.9\\ F{e}^{+3}& 3d^5& 5& Yellow& 5.96\\ C{u}^{+2}& 3d^9& 1& Blue& 1.9\end{array}$

Answer: Option B. -> 2 & 3 
:
B
Because of large number of unpaired electrons in the atoms of transition metals, they have stronger interatomic interaction and hence stronger bonding between atoms resulting in higher enthalpies ofatomization.
Zinc is not regarded as a transition element because zinc atom has completely filleddorbitals (3d${}^{10}$) in its ground state as well as in itsoxidized state.
The high melting & boiling points areattributed to the involvement of greaternumber of electrons from (n-1)d in addition tothe nselectrons in the interatomic metallicbonding. From Sc to Cu,the melting points rise to a maximum at d${}^5$ (Cr) except for anomalous value of Mn and fall regularly as the atomic number increases due to decrease in no. of unpaired electrons.

Answer: Option C. -> enthalpy of atomization is lowest for manganese
:
C
The decrease in metallic radius coupled with increase in atomic mass results in a general increase in the density of these elements. Thus, from titanium (Z = 22) to copper (Z = 29) the significant increase in the density.
For Zn, 1${}^{st}$ ionization enthalpy is higher because it represents an ionisation from the 4slevel.Enthalpy of atomization is lowest for zinc due to absence of unpaired electrons not available for inter-atomic bonding. Cr${}^{2+}$ is reducing as its configuration changes from d${}^4$ tod${}^3$ , the latter having a half-filledt2g level.

Answer: Option A. -> La
:
A
Misch metal is an alloy of rare earth metals with composition :
Rare earth metals – 94.95%
Iron (Fe)– 5%
S,C,Ca,Cl... – Traces
Agood deal of mischmetal is used in Mg-based alloy to produce bullets, shell and lighter flint.

Answer: Option C. -> f –block
:
C
Due to lanthanoid contraction, effective nuclear charge increases, which results in decrease in atomic radii.

Answer: Option A. -> VO+2 < Cr2O2−7 < MnO−4
:
A
This is due to the increasing stability of the lower species to which they are reduced.
$Mn{O}_4^{-}$$\left(M{n}^{7+}\right)$ is reduced to $\left(M{n}^{2+}\right)$ which is having half-filled stable $d^5$ configuration.

Answer: Option B. -> Nickel
:
B
Nickel; Ni + 4CO $⟶$ $\left[Ni\right(CO{)}_4]$(volatile)