Choosing effects when “Divine Dragon – Excelion” is Summoned is a Trigger Effect. Your opponent can chain “Disappear” to remove from play 1 “Divine Dragon – Excelion” in your Graveyard, and you will choose 1 less effect.
If “Chain Destruction” is chained to the Trigger Effect when “Divine Dragon – Excelion” is Summoned, “Chain Destruction” will resolve first, so all your copies of “Divine Dragon – Excelion” will be in the Graveyard and you choose a number of effects equal to the new amount.