You address her using "tu", for it is commonly used for and around family members, when speaking French. So letter "C" would be the correct answer.
Letter "A", would not be used unless you were speaking about yourself, for example: Je vais voir ma mère amis.
(I am going to meet/see my friends mother.) So it does not work for what the query asks.
Letter "B", would not quite fit in this query either, unless speaking in third P.O.V., for example: Elle était en colère contre mon ami. (She was angry with/at my friend.)
And lastly, letter "D", is definitely not useable, in terms of family members. "vous" is commonly used among professionals and other professionals, adminstratives, bosses, and etc. For example: Vous êtes le patron, c’est pourquoi, j’écoute. (You are the boss, therefore, I listen.)
In conclusion, my answer is "A", because it's the most commonly used form of addressing for family members. For example: Tu êtes une mère incroyable! Je t’aime déjà! (You are an incredible mother! I like/love you already!)
Next time: Know that "tu" is the more familiar version of "you", used in social, non-professional situations. And "vous" is for formality, using it mostly in professional situations.
