Which element in group $IA$ (Alkali metals) exhibits the highest chemical reactivity?

The strongest reducing agent among the alkali metals is

Which of the following reactions is not correct?

The correct statement$(s)$ among $I$ to $III$ with respect to potassium ions that are abundant within the cell fluids is/are :
$I.$ They activate many enzymes
$II.$ They participate in the oxidation of glucose to produce $ATP$
$III.$ Along with sodium ions,they are responsible for the transmission of nerve signals

The element with atomic number $55$ belongs to

As per standard reduction potential values,which is the strongest reducing agent among the given elements?