At a particular temperature and pressure for a real gas Van der Waal's equation can be written as:
`(P + a/(V^(2)m)) (V_(m) -b) =RT`

where Vm is molar volume of gas. This is cubic equation in the variable Vm and therefore for any single value of P & T there should be 3 values of Vm. Which are shown in graph as Q, M and L.
As temperature is made to increase at a certain higher temperature the three values of Vm becomes identical. The temperature, pressure & molar volume at point X are called Tc, Pc & Vc for real gas. The compressibility factor in terms of Pc, Vc and T is called Zc.
The expression of Van dcr Waal's constant 'a' can be given as

Options

A. `(27RT_(c))/(16 P_(c ))`
B. `(27(RT_( c))^(2))/(64 P_( c))`
C. `(64(RT_(c ))^(2))/(Pc)`
D. `(27P_( c))/(RTc)^(2)`