Translation:I am nice.
착합니다 means 'is nice, is good' but it's grammatically incorrect to write "I am good" as an answer?
Sorry I do not know the difference in their meaning, but 1 thing i know: grammatical rules.
좋다 is a 'regular' word, while 착하다 is '하다' word.
So 좋다 becomes 좋아 and 좋습니다 etc
While 착하다 becomes 착해 and 착합니다 etc