Which one is a non-causal system?
Answer: A
Because in (a) if you put n = 1 then y(n) = x(2) which shows output depend upon future value.
While in (b), (c) input converted to Past or Present and past.
Note that n cannot be in fraction so for x(n/2) you will put n = 2, 4, ... etc. not 1, 3, 5 .... etc.