Right answer:
B. ¿Por qué no acampamos en las montañas?
This option is the more logical. Since hay una brisa fresca y hace sol is a positive sentence, it is common that a question comes after this. Since it's sunny outside, it's a great time to camp in the mountains with family or friends. Therefore, we can write a complete sentence as follows:
Hay una brisa fresca y hace sol. ¿Por qué no acampamos en las montañas?