A layer of ionosphere does not reflect waves with frequencies greater than 10 MHz; then maximum electron density in this layer is

1.23 × 1011 m−3 1.23 × 1010 m−3 12.3 × 1010 m−3 1.23 × 1012 m−3

1.23 × 1011 m−3

The frequency of electromagnetic wave is  $\nu = 9\sqrt{\text{ maximum electron density}} = 9 \sqrt{d_{max}}$ $\Rightarrow d_{max} = \frac{\nu^2}{81} = \frac{100\times 10^{12}}{81} = 1.23\times 10^{12}$