Positive and Negative are similar to True or False or they are used  for Yes and No.  Positive is True or Yes and Negative is False or No. Here  in your exercise it's  "I like" and "I don't like"

1- I'm not keen on ...  /Positive 

2- I like the sound of ... /Positive 

3- I'm not sure I like the sound of that. /Negative 

4- I'm not crazy about / Negative 

5- That sounds good! /Positive 

I hope that's clearer.

1. Negative 

2. Positive 

3. Is uncertain. Yet to decide if it's neither positive or negative 

4. Negative

5. Positive 

It is clear.

Thanks a lot.

Thank you so much Saqib

 firstly, I want to say that about this discussion, it is an exercise in LEO(Learn English Online). so I need to know meaning of Positive and Negative, are they similar to true and false, respectively? if so,  what is the true of false once?

here, I will try:

1- I don't keen on ...

2- Positive

3- I think that it is positive

4- Positive

5- That sounds are good!

anyone can correct that.