If you picked "A" you might be shocked to learn that most commentators disagree with you. Strangely enough, they can't seem to agree on what the correct answer is, but they all agree on what it's not. Why? Because of the time indicator in the verse which follows that description:

 For the Son of Man will come in the glory of His Father with His angels, and then He will reward each according to his works. Assuredly, I say to you, there are some standing here who shall not taste death till they see the Son of Man coming in His kingdom. Matt 16:27-28  

Jesus told His listeners that some of them would not taste death until they saw the event He had just described. This, according to most commentators, rules out the Second Coming. Although all of the other options have their proponents, the majority favor The Transfiguration as the fulfillment because it immediately follows the prophecy in all three synoptic Gospels. Jesus said that only some of them would not taste death, which implies that most of them would die before the event occurred.  Even by the time of The Ascension only one of the disciples (Judas) had died! Since The Transfiguration occurred only about one week after Jesus made this prophecy, we feel compelled to ask: who died in that week?