The ratio of the mass of 100 billion atoms of magnesium to the mass of 100 billion 24 atoms of lead can be expressed as - $\frac{24}{207}$

Atomic weights are relative masses.

The ratio of the mass of 100 billion atoms of magnesium to the mass of 100 billion atoms of lead can be expressed as.
The mass ratios of Mg to Pb= ratio of atomic masses$=\frac{27}{207}$ (since the number of atoms is the same)
Hence, option B is correct.