Answer: 1. Tu nieto llamó a su primo por teléfono. (por: communication)
2. Viajaron a Bogotá por una semana. (por: duration)
3. Tú volabas para Venezuela cuando te llamaron de la oficina. (para: destination)
4. Mario, para tu abuela es menos incómodo viajar por coche. (para: opinion)
Explanation: A preposition in Spanish can have different meanings depending on the context.
The preposition "por" can be used when you describe the media that you use to communicate: "Tu nieto llamó a su primo por teléfono", stating that he used a telephone to call. It can also be used to describe the duration of something: "Viajaron a Bogotá por una semana" , meaning that they lasted a week in that place.
On the other hand, the preposition "para" can be used to state the destination: "Tú volabas para Venezuela cuando te llamaron de la oficina", meaning that a person was heading to a place . It can also be used to say the opinion of someone: "Mario, para tu abuela es menos incómodo viajar por coche.", stating that for his grandmother, things were different.