Any question answered...

b) as as adult, living with 1 other person, full-time student, i get 25% discount

because

You’ll get 25% off your bill if you count as an adult for Council Tax and live on your own live with people who don’t count as adults for Council Tax.

I.e. it's as if he/she wasn't there. no 50% off I don't think.

b by my understanding

then in what situation does (a) apply?

if your a student living on your own, or with students then its 100% discount (no council tax), so I don't understand the circumstances to qualify for 50%