「」「」
"Who is it?" "It's mom."
Literal
"Honorable-which-person [copula-です-question-か]?" "Honorable-mother [emphasis-よ]"
どなた is the honorific equivalent of 誰 ('who') — used when asking about someone's identity with respect. The reply お母さん uses the honorific お + 母さん (the warm/familial form of 母 'mother'). Note the よ on the reply — feminine-leaning sentence-final particle here, softer than よ after a plain verb.