If $ \int \large\frac{e^x-1}{e^x+1}$$dx=f(x)+c$, then f(x) is equal to

Question

\[\begin {array} {1 1} (a)\;2 \log (e^x+1) & \quad (b)\;\log (e^{2x}-1) \\ (c)\;2 \log (e^x+1)-x & \quad (d)\;\log (e^{2x}+1) \end {array}\]