Volume of blood= Area $\times$ thickness

or Volume $= (100 \; \mu m) \times (100 \; \mu m) \times (0.10\;mn)$

But $1 \;\mu m =10^{-3} mm$

$\therefore 100 \;\mu m= 100 \times 10^{-3} =0.1 \;m$

Volume $= (0.10 \;mm)^3=1.0 \times 10^{-3}$

$1 \times 10^{-3} \;mm$ of blood contain $=30\;RBC$

Therefore 200 mm of blood contain

$\qquad= 30 \times 1.0 \times 10^{3} mm^3 \times 200 \;mm^3= 6 \times 10^{6}$

Hence b is the correct answer.