$$ B^{3+} $$ cannot exist in aqueous solution because of its
B
Small size and large charge
C
Large size and small charge
Correct option is C. Small size and large charge
Being covalent in nature because of small size and large charge it gets hydrolysed in an aqueous solution. In other words, on account of higher charge density i.e. charge/size ratio, it gets hydrolysed forming $$H_{3}BO_{3} $$
$$ 2B^{3+} + 6H_{2}O \rightarrow 2 H_{3}BO_{3} + 6H^{+} $$
Hence, option C is correct.