Answer:
Voy a caminar por las calles.
Step-by-step explanation:
Sentences in which "para" is correct: Voy a la casa para dormir. Voy a ir para el cine. Para mí, un café.
Sentence in which "por" should be used instead of "para": Voy a caminar para las calles. → Voy a caminar por las calles.
For this exercise, it is important to bare in mind the different uses of the prepositions "por" and "para". Generally, "por" is used to explain the cause of something and "para" is used to indicate the purpose of an action.
In the case of the second sentence (Voy a la casa para dormir), the use of "para" is correct because it forms part of an adjunct of purpose. There are also other uses of "para", for example to indicate a direction, as in the third sentence (Voy a ir para el cine.) and to indicate the recipient of an action, as in the last sentence (Para mí, un café).
As regards the first sentence (Voy a caminar para las calles), the preposition "por" should have been used instead of "para" because the sentence indicates the place through which the person is going to walk. This meaning of "through somewhere" requires the use of "por".