Check statement (i): Manganese exhibits $+7$ oxidation state in its oxide.

Manganese forms the oxide $\mathrm{Mn_2O_7}$.

Let oxidation state of Mn be $x$: $2x + 7(-2)=0$ $2x-14=0$ $x=+7$

So, statement (i) is correct.

Check statement (ii): Ruthenium and Osmium exhibit $+8$ oxidation state in their oxides.

Ruthenium forms $\mathrm{RuO_4}$ and osmium forms $\mathrm{OsO_4}$.

For $\mathrm{RuO_4}$: $x+4(-2)=0 \Rightarrow x=+8$

For $\mathrm{OsO_4}$: $x+4(-2)=0 \Rightarrow x=+8$

Hence, statement (ii) is correct.

Check statement (iii): Sc shows $+4$ oxidation state which is oxidizing in nature.

Scandium generally shows the oxidation state $+3$. The $+4$ oxidation state is not shown by Sc in normal chemistry of transition elements.

Therefore, statement (iii) is incorrect.

Check statement (iv): Cr shows oxidising nature in $+6$ oxidation state.

Chromium in $+6$ oxidation state occurs in species like $\mathrm{CrO_4^{2-}}$ and $\mathrm{Cr_2O_7^{2-}}$. These are well-known oxidizing agents, especially in acidic medium.

Therefore, statement (iv) is correct.

Collect correct statements

Correct statements are: $\text{(i), (ii) and (iv)}$

Match with options

This corresponds to Option D.