Final answer:
The sentence with the dangling gerund phrase is 'On completing the grammar quiz, spiking your pencil is acceptable,' because it's missing a clear subject to modify with the gerund phrase.
Step-by-step explanation:
The sentence that contains a dangling gerund phrase is: "On completing the grammar quiz, spiking your pencil is acceptable." This is because the gerund phrase "On completing the grammar quiz" does not clearly state who is doing the completing, resulting in a dangling phrase that lacks a proper subject to modify.
The correct modification would involve directly mentioning the subject that completes the action, such as "On completing the grammar quiz, you may spike your pencil."