Make her the center of your story. Focus on the positive connection the two of you forged as a couple and the bond she and her Daddy created together. Depending on her knowledge about where babies come from, your daughter's questions will arise naturally. When she asks, answer her questions honestly. Let her know the circumstances of your pregnancy and your decision to become a mother despite your partner's departure.
All children love to hear the story of their birth.
Your daughter will feel special because of the positive emphasis you place on her arrival in your life and her Daddy's life. Let her know the special story that is hers. Allow your daughter's questions about her biological father to pace what she is ready to understand and focus upon. Her questions will change as she grows. Eventually, she will no doubt become curious about who this man is.