Electronic configuration of a transition element X in +3 oxidation state is $[\mathrm{Ar}] 3 \mathrm{~d}^5$. What is its atomic number?

The electronic configuration of $\mathrm{Cu}(\mathrm{II})$ is $3 \mathrm{~d}^9$ where as that of $\mathrm{Cu}(\mathrm{I})$ is $3 \mathrm{~d}^{10}$. Which of the following is correct?

Metallic radii of some transition elements are given below. Which of these elements will have highest density?

Generally, transition elements form coloured salts due to the presence of unpaired electrons. Which of the following compounds will be coloured in solid state?

On addition of small amount of $\mathrm{KMnO}_4$ to concentrated $\mathrm{H}_2 \mathrm{SO}_4$, a green oily compound is obtained which is highly explosive in nature. Identify the compound from the following.