An oxidizing agent causes the oxidation of the other atom/element by itself being reduced. In simple words we can state that oxidizing agent gains electrons from the other atom/element i.e. the other atom loses electrons.
From the given options we have to find in which of them electrons are being removed. When the electrons are removed, the number of protons in the atom will be more than the number of electrons. As a result the net charge on the atom will be positive.
First option lists such a change. Initially charge on Al is neutral, 3 electrons are removed and it get +3 charge. This shows that Al is being oxidized, so it needs an oxidizing agent.
