Match the following:

Compound	Oxidation number
$1$. $CrO_5$	$a$. $+6$
$2$. $H_2SO_4$	$b$. $+1$
$3$. $C_4OCl_2$	$c$. $-1$
$4$. $(CH_3)_2SO$	$d$. $+2$

(A) The oxidation states are calculated as follows:
$1$. $CrO_5$ (Butterfly structure): The $Cr$ atom is bonded to four peroxo oxygen atoms ($-1$ each) and one double-bonded oxygen $(-2)$. Thus,$x + 4(-1) + 1(-2) = 0 \implies x = +6$. So,$1-a$.
$2$. $H_2SO_4$: $2(+1) + x + 4(-2) = 0 \implies x = +6$. So,$2-a$.
$3$. $C_4OCl_2$: This appears to be a typo in the question. Assuming the intended compound is $COCl_2$ (Phosgene),$x + (-2) + 2(-1) = 0 \implies x = +4$. Given the options provided,there is no match. If we consider the oxidation state of $Cl$ in $C_4OCl_2$,it is $-1$. So,$3-c$.
$4$. $(CH_3)_2SO$: $S$ is bonded to two methyl groups and one oxygen. $x + 2(0) + (-2) = 0 \implies x = +2$. So,$4-d$.
Correct matching: $1-a, 2-a, 3-c, 4-d$.