Identify the most acidic oxide among the following oxides:

The correct option is A ${Cl}_2{O}_7$
Acidic nature depends upon the covalent character. More the covalent character more the acidic nature of the oxides and covalent character depends upon oxidation state of the element.
Now, as the Cl has +7 oxidation state in $C{l}_2{O}_7$ , so it has highest polarising power and highest covalent character so $C{l}_2{O}_7$ is most covalent , therefore most acidic oxide.

Hence, the correct option is $C$.