The London penetration depth is given by:

Correct option is A

​$$\text{The London penetration depth } (\lambda_L) \text{ is given by:}\lambda_L = \left( \dfrac{m c^2}{4 \pi n e^2} \right)^{1/2}\\\text{Where:}\\\begin{aligned}m &\text{ is the mass of the electron,} \\c &\text{ is the speed of light,} \\n &\text{ is the electron density in the superconductor,} \\e &\text{ is the charge of the electron.}\end{aligned}$$​​