What is the expression for Fourier series coefficient ck in terms of the discrete signal x(n)?

• A. ( frac{1}{N} sum_{n=0}^{N-1}x(n)e^{j2 kn/N} )
• B. (N sum_{n=0}^{N-1}x(n)e^{-j2 kn/N} )
• C. ( frac{1}{N} sum_{n=0}^{N+1}x(n)e^{-j2 kn/N} )
• D. ( frac{1}{N} sum_{n=0}^{N-1}x(n)e^{-j2 kn/N} )