Which of the following cannot function as an oxidising agent?

An oxidising agent is a substance that oxidises other substances involved in the reaction by gaining or accepting electrons from them.

So the lowest oxidation state substance will go to the higher oxidation state, which means it reduces others. Hence, it acts as a reducing agent. $-3$ is the lowest oxidation state for nitrogen. Hence, it only acts as a reducing agent.

Hence option A is the answer.