At low pressure, the Vander Waal's equation is expressed as

• A

  ${\mathrm{pV}}_{\text{m}}=\mathrm{RT}+1$
• B

  $\mathrm{Z}=\frac{\mathrm{p}{\mathrm{V}}_{\mathrm{m}}}{\mathrm{R}\mathrm{T}}=1+\frac{\mathrm{b}}{\mathrm{R}\mathrm{T}}\mathrm{p}$
• C

  $\mathrm{Z}=\frac{\mathrm{p}{\mathrm{V}}_{\mathrm{m}}}{\mathrm{R}\mathrm{T}}=1-\frac{\mathrm{a}}{\mathrm{R}\mathrm{T}}$
• Correct answer
  D

  $\mathrm{Z}=\frac{{\mathrm{pV}}_{\text{m}}}{\mathrm{RT}}=1-\frac{\mathrm{a}}{{\mathrm{RTV}}_{\text{m}}}$

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