What does orthoboric acid contain?

Which of the following elements form dimeric halides?

In the following reaction,assign the correct category for the underlined atom in the product of complete hydrolysis at $R.T.$:
$\underline{I}Cl_3 + H_2O \longrightarrow HIO_2 + HCl$

Consider the following reactions (not balanced):
$BF_3 + NaH \xrightarrow{450 \ K} X + NaF$
$X + H_2O \longrightarrow Y + H_2 \uparrow$
The correct statements about $X$ and $Y$ are:
$I$. $X$ is an electron-deficient molecule
$II$. In $X$, $B-B$ bond is present
$III$. $Y$ is a weak tribasic acid
$IV$. $Y$ acts as a Lewis acid

The Lewis acid strength of $BBr_3$,$BCl_3$,and $BF_3$ is in the order:

Which of the following is the most basic oxide?