Select the principal group when the following groups are present in a molecule.
`-Cl ,-OH ,-NO_2 , -CHO`

Explanation

To determine the principal functional group among the given groups (-Cl, -OH, -NO2, and -CHO), we need to follow these steps:

### Step 1: Identify the Functional Groups
The functional groups present in the molecule are:
- -Cl (Chloro)
- -OH (Hydroxyl)
- -NO2 (Nitro)
- -CHO (Aldehyde)
...