In which of the following ionic radii of chromium would be smallest

Among $Co ^{3+}, Ti ^{2+}, V ^{2+}$ and $Cr ^{2+}$ ions, one if used as a reagent cannot liberate $H _{2}$ from dilute mineral acid solution, its spin-only magnetic moment in gaseous state is $......B.M$. (Nearest integer)

$(a)$ Transition metals can act as catalysts because these can change their oxidation state. How does $Fe(III)$ catalyse the reaction between iodide and persulphate ions ?

$(b)$ Mention any three processes where transition metals act as catalysts.

The $3d$ metal ions form coloured compounds because the energy corresponding to the following lies in the visible range of electromagnetic spectrum

The $d-$ block element which is a liquid at room temperature, having high specific heat, less reactivity than hydrogen and its chloride $(MX_2)$ is volatile on heating is

For the following Assertion and Reason, the correct option is

Assertion : For hydrogenation reactions, the catalytic activity increases from Group $5$ to Group $11$ metals with maximum activity shown by Group $7-9$ elements.

Reason : The reactants are most strongly adsorbed on group $7-9 $ elements.