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You may attach the Aura to the token only if it can legally enchant that token. For example, if the enchantment that entered the battlefield is an Aura with "enchant land," it won't become attached to that token.
If you cast an Aura targeting an opponent or a permanent controlled by an opponent, you control that Aura when it enters the battlefield. The ability of Ajani's Chosen will trigger.
You may attach the Aura to the Cat token only if the Aura and the token are both on the battlefield when the triggered ability resolves. If the Aura leaves the battlefield before the ability resolves, it won't become attached to the token. If the token leaves the battlefield before the ability resolves, the Aura won't become attached to that token.
The ability doesn't trigger until the enchantment enters the battlefield. This means that, for example, you can't cast an Aura with enchant creature if there are no legal targets available, hoping to enchant the Cat token. Ajani's Chosen itself, however, may be a legal target for the Aura.
