A sample of municipal water contains one part of urea (molecular wt $=60$ ) per million parts of water by weight. The number of urea molecules in a drop of water of volume $0.05ml$ is

 we know that 1ppm = 1mg of urea in 1000ml of water.

Weight of urea in 0.05ml of water = $\frac{1\times 0.05}{1000}=5\times {10}^{-5}mg=5\times {10}^{-8}g$

Therefore number of moles = $\frac{5\times {10}^{-8}g}{60}=8.33\times {10}^{-10}moles$

Thus number of molecules$= 8.33\times {10}^{-10} moles \times 6.022 \times {10}^{23} = 5 \times {10}^{14}$ molecules. 

Hence the correct option is (B).