Final answer:
The correct verb form to complete the sentence is 'hará'.
Step-by-step explanation:
The correct verb form to complete the sentence 'Hoy hace frio pero mañana un buen día' is OC. hará. The verb 'hará' is the future tense form of the verb 'hacer' which means 'to do' or 'to make' in English. In this context, it means 'will be'.
Learn more about Spanish verb forms