Answer
All of them.
Work Step by Step
a) \begin{equation}\sum_{k=1}^{6}2^{k-1}=2^{1-1}+2^{2-1}+2^{3-1}+2^{4-1}+2^{5-1}+2^{6-1}=2^{0}+2^{1}+2^{2}+2^{3}+2^{4}+2^{5}=1+2+4+8+16+32\end{equation}
b) \begin{equation}\sum_{k=0}^{5}2^{k}=2^{0}+2^{1}+2^{2}+2^{3}+2^{4}+2^{5}=1+2+4+8+16+32\end{equation}
c)\begin{equation}\sum_{k=-1}^{4}2^{k+1}=2^{-1+1}+2^{0+1}+2^{1+1}+2^{2+1}+2^{3+1}+2^{4+1}=2^{0}+2^{1}+2^{2}+2^{3}+2^{4}+2^{5}=1+2+4+8+16+32\end{equation}
a, b and c express 1+2+4+8+16+32 in sigma notation.