根据《地规》5.2.4，$f_{spa}=f_{spk}+\eta_d\gamma_m(d-0.5)$，$\eta_d=1.0$，$\gamma_m=\frac{1\times18.6+0.8\times8.9}{1.8}=14.3\mathrm{kN/m^3}$，$d=1.8\mathrm{m}$，$250=f_{spk}+1.0\times14.3\times(1.8-0.5)=f_{spk}+18.59$，$f_{spk}=231.4\mathrm{kPa}$。根据《地处规》7.1.5，$f_{spk}=\lambda m\frac{R_a}{A_p}+\beta(1-m)f_{sk}$，$A_p=0.1256\mathrm{m^2}$，$f_{sk}=80\mathrm{kPa}$，代入得$231.4=0.9\times m\times\frac{680}{0.1256}+1.0\times(1-m)\times80$，$0.9\times680/0.1256=4874.5$，$231.4=4874.5m+80-80m$，$151.4=4794.5m$，$m=0.0316$。条形基础单排桩，$m=\frac{A_p}{b s}$，$0.0316=\frac{0.1256}{2s}$，$s=\frac{0.1256}{2\times0.0316}=1.99\mathrm{m}$，选A。