The correct answer is B: in. The phrase "interested in" is a common and correct idiom in English, used to express a feeling of curiosity or concern about something, or a desire to learn more about it. When you are "interested in" an activity, a subject, or a person, it means you find it engaging and want to dedicate attention or effort towards it. In this sentence, "learning French" is the activity that captures her interest.
- A: for is used to indicate purpose or recipient, e.g., "a gift for you," not interest.
- C: at is typically used for specific locations or times, e.g., "look at the picture," or "at 5 o'clock."
- D: on is used for surfaces or specific topics, e.g., "the book is on the table," or "a discussion on politics."
The preposition "in" correctly links the feeling of interest to the object or activity of that interest. It's a fixed expression that native speakers use naturally, and understanding such idiomatic phrases is key to mastering English grammar and usage. Therefore, "interested in learning French" is the grammatically sound and natural-sounding construction.