Which of the following is the sum of divisors function σ(pk)\sigma(p^k)σ(pk)?
pk+1−1p−1\frac{p^{k+1}-1}{p-1}p−1pk+1−1
pkp^kpk
k⋅pk \cdot pk⋅p
pk+1−1p^{k+1} - 1pk+1−1