Step 1: Reinterpret the given product:
$V\times d$ has units of cm$^3 \times$ g/cm$^3$ = g. It is the mass of a single cell, $6.65\times10^{-23}$ g.
Step 2: Count the cells:
Cells $= 0.5 \text{ g} / (6.65\times10^{-23} \text{ g per cell})$.
Write this as $\dfrac{5\times10^{-1}}{6.65\times10^{-23}} = 0.7519\times10^{22}$.
Moving the decimal point gives $7.518\times10^{21}$.
Step 3: Match:
Only option (B) agrees with this value. The mass of the sample is so much more than the mass of one cell that the count is of the order $10^{21}$.
Final Answer:
Number of unit cells $= 7.518\times10^{21}$, option (B).
\[ \boxed{7.518\times10^{21} \text{ (B)}} \]