"Dad is speaking to her."
Translation:אבא מדבר אליה.
I entered אבא מדבר לה and was marked wrong. What's the difference between ל and אל?
It's a bit tricky. A lot of times אל and ל can be used interchangeably; sometimes they can't. I'm not sure if there's a general rule, but I'll try to explain the difference as I see it:
1) In verbs that have a sense of direction or movement towards something/someone, literally or figuratively - you can use either אל or ל, unless you use a pronominal suffix, in which case you have to use אל. Examples - ללכת אל החנות/ללכת לחנות but only ללכת אליו (NOT ללכת לו). Some other verbs - לשחות (swim), להגיע (reach/come), לבוא (come), להתגעגע (to miss), לשים לב (to pay attention/notice), להתקשר (to call) and many others.
2) In verbs that have a sense of giving something to someone, you can only use ל. Examples - לתת לאמא (NOT לתת אל אמא), or להשכיר (rent), להפריע (bother), לשלם (pay), לעזור (help).
In this case: "לדבר" falls into the first category. I think this is because "לדבר אל" is somewhat like saying "talking to your direction", whereas "לדבר עם" is to have a conversation with someone.