A. Her interest in her father might be piqued because she's starting to notice the doting dads in books and videos and at her friends' houses.
But kids are tougher than you think, and to them, normal is whatever environment they're being raised in. Your child understands that you're her sole parent and is probably just fine with that for now. Still, she has the right to know about, if not actually know, both of her parents. It won't break her heart to hear the basic facts.
Explain that her father doesn't live with you because Mommy and Daddy had grown-up problems that they just couldn't work out. A neutral stance on Dad, from the very first conversation, will allow your daughter to form her own opinion of her father, based on his actions (or lack thereof), and prevent you from morphing into the witch who made him go away. Say that you don't know why he doesn't come to see her but that you do know everyone in the family loves her. You might also try telling her that someday, when she's older, she may meet her dad in person, and then she can ask him anything she wants.
At that point, she may well try to find and meet him, and even ask for your help. Or she may refuse to acknowledge him altogether, out of anger or indifference. Don't try to minimize her emotions; just stand by your girl, affirm her right to all of her feelings about her dad, and try to answer any questions she may have. You may regret that she has this poor excuse for a father, but you can't erase him from her life. Besides, amazing creations can come from even the most flawed ingredients.