As Kramze47 said, every option should be accepted.
Det can, also as mentioned before, refer to both neuter- and common gender nouns, when it means "it"
If you do not know the gender of the noun, you use the neuter, det.
So if the sentence started out with "den...", you would know it was min because using other den shows that you know the gender of the noun (in this case common n-word)
Disclaimer: I only know what I've learned so far in this course. The grammar tips and notes for this section are great, though.
"Det" is a clue. You have to use "mit" because what is being referred to is a neuter word. We know this because "det" was used instead of "den".
It could be en æble. æble is a neuter word so when referring to it you say det.
So, "det er mit."
"It (æblet) is mine."
"Den er min."
"It (hesten) is mine."