What does "x(n) is complex" mean? (DSP)
It means that $x(n)$ and $y(n)$ can be complex numbers rather than just real numbers. As a result there is no conjugate symmetry in the Frequency domain for these signals. Note that part (c) the asterisk in $x^*(n)$ usually means to take the complex conjugate. If $x(n)$ and $y(n)$ were real, i.e. not complex, then (b) and (c) would be equivalent.