I wrote down: "He is a mouse" which was incorrect. But how come in the correct answers are written that: " he's a mouse" is correct?
Correct solution: • He's a mouse. • He has a mouse.
"He's" here is a rare contraction of "he has". It's rare exactly because it is easily confused with the contraction of "he is".
Apparently, "he's" is the abbreviation of "he has" just like "I've" stands for "I have". However, I think you cannot contract it this way unless it's an auxiliary verb.
Darn... well, the algorithm knows that there are contexts in which "He has" can be shortened to "He's" (He's never been to India), but it doesn't actually distinguish contexts, so it accepts it everywhere, I'm afraid :/ Sometimes even suggests it. I hope that it will be finally fixed... but we can't do anything about it on our side, that's for the developers.