If we look at the part of the sentence "when a football practice was over" we can say that it is a recurring action periodically and it represents a routine, but that's not a phrase because it is stated specifically, what is the issue. "I stopped" also suggests a concrete action, therefore, it's not a phrase, because it explicitly indicates what has been done. What is the phrase in this sentence is a cluster of words without a finite verb, and having that in mind, the correct answer is "riding a skateboard".
