Which formula represents the rest mass of an electron?

• A
  $m_{rest} = m \sqrt{1 - (\frac{v}{c})^2}$
• B
  $m_{rest} = \frac{m}{\sqrt{1 - (\frac{v}{c})^2}}$
• C
  $m_{rest} = m \times (1 - (\frac{v}{c})^2)$
• D
  $m_{rest} = \frac{\sqrt{1 - (\frac{v}{c})^2}}{m}$