Metal carbonyls can have formula $M\left(CO{)}_x\right)$, where $x=$ number of $CO$ units coordinated to metal $M$.

What is the formula of the carbonyl of $Fe\left(26\right)$ if $EAN$ of $Fe$ in metal carbonyl is $36$?

The correct option is D 5
The effective atomic number :

$EAN=Z-X+Y$

$Z=$ atomic number of iron which is $26$.

$X=$ number of electrons lost during formation of metal ion from its atom which is 0.

$Y=$ number of electrons donated by the ligands which is $2x$ as there are $xCO$ ligands and each one donates 2 electrons.

$EAN=26-0+2x=36$

$2x=10$

$x=5$

Thus, the metal carbonyl is $Fe(CO{)}_5$.

Hence, the correct option is D.