According to <u><em>SOS,</em></u> the answer is
"In addition to quiet reflection, Memorial Day is also a time for boisterous gatherings."
Hope this helps!
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 <em>correct</em>.
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!