Sum of series $\;\frac{1}{2}+\frac{3}{4}+\frac{7}{8}+\frac{15}{16}+.....\;is:$

Question

$(a)\;\frac{n+1}{2^{n+1}}\qquad(b)\;\frac{n}{2^n}\qquad(c)\;n+1-\frac{1}{2^n}\qquad(d)\;n-1+\frac{1}{2^n}$