spanish 2 fill in the blank questio with ser
Anita y Ud. is the 2nd plural form so you use sois (vosotros sois) which is used in the "standard" Spanish from Spain. Note that other Hispanic countries use the 3rd plural form in this case: son (ustedes son). So the correct answer may depend mostly on whether your book and/or teacher follows the Spanish from Spain of from other Hispanic countries (South America/Central America/Caribbean).
if you see in the table of verb ser vosotros sois then the correct answer is sois.