Juanita, mi hermanastra y yo pensamos competir en Buenos Aires en el invierno.
Explanation:
Juanita, mi hermanastra y yo pensamos competir en Buenos Aires en el invierno. In English this means: Juanita, my stepsister and I are thinking about competing in Buenos Aires in the winter. Pensamos is the verb "pensar" conjugated in the first person of the plural. "Piensamos" is grammatically incorrect, it doesn't exist. "Pienso" is the conjugation in the first person singular, and "pensó" is the conjugation in the third person singular. Since the sentence refers to "us" the only option possible is pensamos.
