1. Use Einstein’s mass-energy relation:

$E = mc^2$

For $1\,\text{g}$ of substance,

$m = 1\,\text{g} = 10^{-3}\,\text{kg}$

Take

$c = 3 \times 10^8\,\text{m/s}$

So,

$E = 10^{-3}(3 \times 10^8)^2$

$E = 10^{-3} \times 9 \times 10^{16}$

$E = 9 \times 10^{13}\,\text{J}$

2. Convert joules to electron volts.

We use

$1\,\text{eV} = 1.6 \times 10^{-19}\,\text{J}$

Hence,

$E = \frac{9 \times 10^{13}}{1.6 \times 10^{-19}}\,\text{eV}$

$E = \frac{9}{1.6} \times 10^{32}\,\text{eV}$

$E = 5.625 \times 10^{32}\,\text{eV}$

3. Convert eV to MeV.

Since

$1\,\text{MeV} = 10^6\,\text{eV}$

therefore,

$E = 5.625 \times 10^{26}\,\text{MeV}$

$E \approx 5.6 \times 10^{26}\,\text{MeV}$

4. Match with the options.

Option A is:

$5.6 \times 10^{26}\,\text{MeV}$

So the correct answer is A.