Is there a difference between these sentences?

[Offroad]

Hello guys,

If someone ask about something, then one replies:

a) I think so
b) I think
c) I guess so
d) I guess

So, could you tell me if/whether there are differences between a) and b) or c) and d).

Thank you in advance.

 

[David L.]

a,c, and d are possible replies.

 

[Offroad]

Thank you for your reply David, but I think option c) can be used as well/too.

Someone asks:
- What's it? Did you hear?
- yeah, that's the doorbell, I think.

- Is it your friend?
- I think so / I guess / I hope so

 

[David L.]

??
read my post again

A, C and D

 

[Offroad]

sorry, I meant option b) can be used too. Agree?