Right answer:
c. tráigalo
This is given in the imperative mood. This mood is used when telling someone to do something in a direct manner. Put another way, sentences in the imperative mood are commands. Tráigalo is the correct conjugation of the verb traer in formal form and for singular nouns. Put simply, you are telling someone to bring ice cream and usted makes the sentence to be formal because usted is the subject pronoun for the second person singular (formal you)