It's because the person in the sentence is talking to a female (Maha). Your (of a female: ~ik) dog, كَلْبِك.
In the cases when the interlocutor is a male is used a Your (for a masculine interlocutor, for example, George: ~ak): كَلبَك.
But what I've been reading in some other comments, what we're learning in Duolingo is a mixture of dialects, not the Modern Standard Arabic, so surely there are other rules that we haven't learnt.
Actually... in the sentence above, from many native Arabic-speakers, they say the phrase as : "kalbuki" كلبُكِ (Formal) or just "kalbuk" (informal) -and not "kalbik"- to a female, e.g., Maha.
Then, it will be "kalbuka" كلبُكَ (Formal) or "kalbuk" (informal) -and not "kalbak"- to a male, e.g., George.
These are the Standard Arabic Grammar for the nominative case (like the sentence above).
I don't know why Duo do this (it seems the contributors want to simplify the lessons so they are not too difficult?) but some native member has explained that "kalbak" كلبَك (for a dog's male owner) and "kalbik" كلبِك (for a dog's female owner) in the nominative case are from a local dialect.
Nb: some native members also have reported this as an error as the title course is "Arabic language" that isn't specific to a local dialect.