Which of the following halides is least stable and has doubtful existence?

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Carbon forms two important oxides $- CO$ and $CO_2$. $CO$ is neutral whereas $CO_2$ is acidic in nature.
Reason $R$: $CO_2$ can combine with water in a limited way to form carbonic acid,while $CO$ is sparingly soluble in water.
In the light of the above statements,choose the most appropriate answer from the options given below:

Which of the following is not a monomer for a high molecular mass silicone polymer?

Discuss the reactivity of group $14$ elements towards water and halogens.

Match the species given in Column-$I$ with the properties mentioned in Column-$II$.

Column-$I$	Column-$II$
$A$. $BF_4^-$	$1$. Oxidation state of central atom is $+4$
$B$. $AlCl_3$	$2$. Strong oxidizing agent
$C$. $SnO$	$3$. Lewis acid
$D$. $PbO_2$	$4$. Can be further oxidized
	$5$. Tetrahedral shape

Give the chemical reaction for preparing pure $CO$.