Answer
B
Work Step by Step
$HCO_2^-(aq)+H_2O(l)\rightarrow HCO_2H(aq)+OH^-(aq)$
$K_b=[HCO_2H][OH^-]/[HCO_2^-]$
$[Na^+]=0.10\ M, [HCO_2H]=[OH^-]=x, [HCO_2^-]=0.10-x$
$5.6\times10^{-11}=x\dot{}x/(0.10-x)$
$x=2.37\times10^{-6}\ M$
$pH=14+log([OH^-])=8.37$
$[HCO_2^-]=0.10\ M$