Answer: In the sentence provided, "Rolling on the lawn" is the participial phrase.
Step-by-step explanation: A participial phrase is made up of a present or past participle and modifiers. It always functions as an adjective. Moreover, in general, participial phrases are found at the beginning of a sentence. For example, in the sentence provided, "Rolling on the lawn" is the participial phrase since it is made up of a present participle ("rolling") and its modifier ("on the lawn"). Furthermore, "rolling on the lawn" is acting as an adjective as it is describing the noun "Duncan".