Your answer is the third option, he was golfing.
he will be golfing ⇒ "will be" indicates it has not yet happened, or is happening in the future, so this option is incorrect.
he is golfing ⇒ "is" indicates it is happening right now, or in the present, so this option is incorrect.
he was golfing ⇒ "was" indicates it already happened, or it happened in the past, so this option is correct.
he will have been golfing ⇒ is incorrect grammar usage, therefore making it incorrect as well.
I hope this helps and have a great rest of your day!