$B^{3+}$ ion cannot exist in an aqueous medium because of its:

The correct option is C Small size and large charge
On account of higher charge density i.e. charge/size ratio $B^{3+}$ gets hydrolysed to $H_{3}B{O}_3$.

$2B^{3+}+6H_{2}O\to 2H_{3}B{O}_3+6H^{+}$