How to summon tnt to a specific player’s location
Put this in your command block:
execute <player> ~ ~ ~ summon <entity>
Whatever the player's name is, put that on <player>, and summon any <entity> given in the command.
You can try something else, however. Changing the coordinates where the player is standing by y + 75 will make it look like they nearly got nuked.
Simply:
execute <player> ~ ~75 ~ summon tnt