If $\;a_{1}=1\;,a_{n+1}=2a_{n}+1$ then , $\;a_{n+1}$ equals

Question

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