The correct option is
D is very pretty
The question 'what is Mary like?' indicates an enquiry about Mary's appearance. In other words, the question is asking how Mary looks. Thus, the answer must indicate a response to the question and must thus, describe how Mary looks.
Option A is incorrect. 'is very well' describes how Mary is feeling and not how she looks. Thus, it does not convey the correct meaning and is incorrect.
Option B is incorrect. 'likes ice-cream' indicates what Mary likes and not how she looks. Thus, it does not convey the correct meaning and is incorrect
Option C is incorrect. 'is like tall' describes a feature of Mary's. Further, 'like tall' makes no sense. One cannot be like 'tall' and so, it does not make sense.
Option D is correct. 'is very pretty' indicates how Mary looks. It says that Mary looks very pretty. Thus, it conveys the correct meaning and is the correct answer.