Which of the following is a single valence radical?  

Key Points

• Chloride (Cl-) is a monovalent (single-valence) radical as it has a charge of -1.
• Hydroxide (OH-) and nitrate (NO₃-) are both polyvalent radicals.
• The Hydroxide group has a -1 charge and therefore it might seem to be a monovalent radical, but it is usually seen in the context of basic compounds which makes it polyvalent.
• Nitrate has a -1 charge but it consists of multiple atoms.
• So, only Cl have single valence radical.

Additional Information 

• Chloride (C^l-): Chloride is a single-valence negative ion (anion). It is formed when a chlorine atom gains one electron. The electron configuration of a chloride ion is the same as the noble gas argon, making it stable.
• Hydroxide (OH^-): The hydroxide ion consists of a hydrogen atom bonded to an oxygen atom, carrying a negative charge overall. This brings it a total of 1- charge, making it a single-valence anion. This ion is a common component of basic compounds, and when dissolved in water, it contributes to the solution's alkalinity.
• Nitrate (NO₃^-): The nitrate ion is a polyatomic ion composed of one nitrogen atom covalently bonded to three oxygen atoms. The whole assembly carries a charge of -1. Although its overall charge is -1 similar to chloride and hydroxide, it is not a single-valence radical. It has resonance structures, which means the negative charge is spread over the entire ion making it a polyvalent (multivalent) radical.

Conclusion:

So, Chloride is single valence radical.