The correct answers are Having added too much frog extract, the wizard accidentally blew up the cauldron. and Startled by the sudden explosion, the wizard dropped his wand.

Having added... is a perfect participle clause correctly showing an action that happened before the main clause. The subject ("the wizard") matches the person who added the extract.

Startled by... is a past participle clause correctly showing a passive meaning (the wizard was startled).

"Added too much..." is incorrect because the cauldron didn't add the extract (this is a dangling participle).

"Startling by..." is incorrect because "startling" has an active meaning, but the context requires a passive meaning ("startled"). Additionally, the wand wasn't the one being startled!