In the sentence provided, the phrase that completes the thought is "playing the harmonica." This is because the sentence describes Grandpop engaging in a physical activity that leads him to sweat and be out of breath. Out of the options given, playing the harmonica is the most suitable activity that would not only involve physical exertion but also be a plausible reason for him to end up sweating and out of breath.
Playing the harmonica requires using both hands and coordinating breathing to produce music, which can be physically demanding, especially for an extended period. This physical activity aligns with the idea of Grandpop exerting himself until he reaches a point of being sweaty and out of breath.
