Sum of n terms of the series $\large\frac{1}{4}+\large\frac{7}{16}+\large\frac{37}{64}+\large\frac{175}{256}$+... is
$(a)\;n-3^{-n}+1\qquad(b)\;n-4^{-n}+3^{-n}\qquad(c)\;n-3(1-(\large\frac{3}{4})^n)\qquad(d)\;n-((\large\frac{4}{3})^n-1)$