For simple cubic crystal edge length is expressed as

• Correct answer
  A

  $\mathrm{a}=2 \mathrm{r}$
• B

  $a=\frac{r}{2}$
• C

  $a=\frac{r}{2}$
• D

  $\mathrm{a}=\frac{\mathrm{r}}{\sqrt{2}}$

Hence, the correct answer is option: 'A'.