"My mother is a secretary."
Translation:Mia madre fa la segretaria.
When you use a diminutive or superlative (little sister - sorellina, big brother - fratellone) then you must use the "la" in front (la mia sorellina) - so the same with "mamma" (like mommy) - "la mia mamma." But my sister and my mother (no diminutive) don't require the "la" so it's just, "mia sorella" or "mia madre."
lkamjh: One of the correct answers was "mia madre e una segretaria" so perhaps it's because you put in the 'la" and it's not needed for family members. I tried to be clever and put "fa una segretaria" but that was also wrong because it should have been "fa la segretaria", though unfortunately I don't know why!