When Aven Surveyor enters the battlefield, choose one —
• Put a +1/+1 counter on Aven Surveyor.
• Return target creature to its owner's hand.
11/24/2014 You choose which mode you’re using as you put the ability on the stack, after the creature has entered the battlefield. Once you’ve chosen a mode, you can’t change that mode even if the creature leaves the battlefield in response to that ability.
11/24/2014 If a mode requires a target and there are no legal targets available, you must choose the mode that adds a +1/+1 counter.