I think it should be sp2
It should be sp² because it has 1 lone pair and 2 pie bond
The short answer in sp2, but for a detailed solution and how it is made follow along.
Chlorine has 7 electrons in the valence shell, and the structure involves bonding of the chlorine atom with two oxygens with double bonds a total of 4 electrons are used up in this process and creating two double bonds.
3 electrons remain two of which form a lone pair and one electron is left out and is called the odd electron.
Extra Tip -- This odd electron gives molecules the property to dimerize but ClO2 is an exception it does not dimerize when cooled (Important for JEE). Some molecules like NO2 show this dimerizing property