Fill in the blank in the following sentence with the appropriate verb form below.
mi hermana muy bien.
a. cocinas
b. cocinamos
c. cocino
d. cocina

D.) Cocina because taking in the feminine verb as Hermana means sister. Option A.) Cocinas is plural and in third person not making much sense. Option B.) Cocinamos would not be the correct answer as cocinamos means you and your sister are cooking and in the question it only mentions her sister. and finally option C.) Cocino is in first person and they are not talking about themselves they are talking about the sisters cooking. 

D. Cocina
Why? Is talking about a girl, so is not c, is talking about one person, so is not option b and finally is talking in present, so is not a.

Almost every country has citizens that speak spanish but there is no country i know of that every one speaks spanish hope this