Solution

(A) It is a weak monobasic Lewis acid. Soluble in water and in aqueous solution the boron atom completes its octet by accepting $$ OH^{-} $$ from water molecules :

$$ B(OH_{3}) + 2H_{2}O \rightarrow [B(OH)_{4}]^{-} + H_{3}O^{+} $$

(B) Only $$Tl^{+3} $$ acts as an oxidsing agent on account of inert pair effect.

(C ) In the solid state, the $$ B(OH)_{3} $$ units are hydrogen bonded together into two dimensional sheets with almost hexagonal symmetry.



(D) $$ B(OEt)_{3}$$ imparts green colour to the burner flame.

Hence, option C is correct .